Getting the Right Image Size for eBook

Making sure you have the right image size for ebook can seem daunting, but it's really not all that complicated if you know a few simple things about resolution and saving images for web.

I use Jutoh to create ebooks and do all the formatting directly in the Jutoh program (as opposed to formatting in Word or another word processing program and then transferring to Jutoh), but recently a friend of mine switched to Jutoh and was surprised at the sudden increase in her finished file size. That’s because many programs like Calibre will size down images without alerting you, while Jutoh doesn't. Sizing down is great, but in order to maintain control over the final product, it’s important to understand image resolution and quality and size the image down yourself to get the best image size for ebook.

Why file size matters

Many platforms limit the ebook file size, and Amazon actually charges you 15 cents per MB, so if a book that should be less than 1 MB turns out to be 10 MB as a Kindle mobi file, that’s going to affect your bottom line. Cover images, jpeg chapter images, scene breaks, author picture, and promo images all add to the final file size.

Note that the size measurements I'll be referring to here are KB and MB. KB stands for kilobyte (equal to 1024 bytes). MB means a megabyte. It takes 1024 KBs to make one MB (not 1000 as many might think).

Embedding a font vs using an image

Embedding a font can help lower file size if you have a large book with a ton of special chapter headings, but unless you know how to strip the font down to the very basics, often embedding the font will take just as much space as images, and some ereaders won’t show the font anyway, so your hard work goes to waste on those devices.

Images will show across all ereaders, and you can experiment with your images to determine a size that will not compromise your quality. But images do have a few disadvantages over embedded fonts.

Potential pitfalls of using images

There are some things you need to be aware of, especially if you are using images to "fancy up" chapter headings, i.e., images that are supposed to look like text.

The JPEF format does not support transparency, and most ereaders only support JPEG. So if the user changes the ereader's background from white to some other color, the white background of your image will show.

While an embedded font will size with the user's font size choice, images will not. So as the user changes the font size on the device, the text will grow bigger or smaller, but images will not. In the case of images with no words, such as dingbats used for scene breaks or to embellish chapter headings, or your author picture, this may be a desirable outcome. But for chapter headings it might not be.

Images are very much affected by the device's resolution. Let's say you create a fancy chapter heading as a 600 pixel wide image. It will not look consistently the same across all devices. For example, the Kindle Fire only supports a 600 pixel resolution across the width, so your image will take the entire width, but on HD models, all of which support at least 800 pixels across the width, that will not be the case (click here for a list of Kindle device specifications.)

Of course, you can tell the device that the image should take 100% of the width (or any %, for that matter), but then if proportions are to be maintained the device will also have to change the image's height.

The take away is that you should test your book on devices of different sizes and screen resolutions to make sure the look is acceptable to you on every device.

How to get the right image size for ebook

The words you most need to remember here are "resolution" and "save for web." For this tutorial, I am using Photoshop, but any photo editing program should have similar options.

For book covers that appear inside the ebook (not the thumbnail that vendors show), I recommend choosing a resolution of 1400 pixels wide for inside your ebook. This is because iTunes, and third parties who distribute to iTunes, want cover images with at least that width. (There is also a maximum limit that varies between vendors.) If you are not uploading to iTunes but only other vendors, you can instead choose 1000 pixels wide. For Amazon, you will not put the cover inside your ebook, of course, but they'll put the image you upload into the finished mobi file for you, so you can still use the same size-reducing techniques for the cover there. Manually changing the size of your cover may actually make your cover look better on Amazon, Barnes & Noble, Kobo, and other platforms because quite often the different vendor reduction programs don't do as good a job.

Resizing an image

First let's experiment with a 6 X 9 inch image, which, at 300 dpi, has a resolution of 1800 x 2700 pixels. The current size of this photo is 3.32 MB. Way too large. You definitely don't want a cover that big for ebook. Also keep in mind that different photos and different designs will run higher or lower depending on the elements you've included and the photo itself.

For the moment, don't worry about inches or dpi. All you need to worry about is the width in pixels. To change the resolution, choose Image>Image Size.

Make sure the dimensions are measured in pixels and change the width to 1400. For the pixel options to be available, you must select the checkbox Resample Image. Be sure that the width and height ratios are locked so your picture doesn't distort (check the box Constrain Proportions).

Even just sizing the photo down to 1400 pixels wide took it down to about 2 MB.

Saving for web

But that's not nearly small enough. Because just that single image alone will cost you 30¢ to deliver on Amazon. Now it's time to tinker with the image quality. Choose File>Save For Web and you get something like this cool window. This is where you'll experiment to see what looks good.

Look at the yellow highlight in the upper left corner. Here, you can switch between your original document and the one you're sizing (optimized) to see the difference in appearance. Then look at the highlighting on the upper right. To look good, some images will need high/60 quality and will turn out to be about 500 KB. But this particular image looks just as good notched down to medium/30, and is now a small 178.9 KB (see highlight on lower left side). Since there is no text and a lot of empty space in the photograph, we maybe could have gone under 100 KB. But be careful not to go too low on the quality; if it looks pixelated on your screen, readers will notice—and your product will not look professional.

You may have to play with the amounts before you get them just right. And remember to check your ebook on your ereader to make sure the final image looks great. For smaller images, or additional images in the book, you definitely do not have to start with the 1400 pixels wide resolution. Dingbats, for example, will be much smaller. Mine generally range from 200 to 400 pixels wide.

Basic targe size for individual images

A key size for non-cover, full-sized images is 100 KB, but again this will vary widely. Images with a lot of color and detail (like in some picture books) may require more, while some might be a fraction of that size. In my fixed layout picture book, I Don't Want to Eat Bugs under the name Rachel Branton, the full-size interior images range from 54 KB to 153 KB. These sizes are still larger than images used by many picture books, but the pictures cover the entire screen, and I wanted the quality to be high. I was happy to get the book under 4 MB.

A single small dingbat cropped to size and adjusted for quality using the "save for web" option might run only 2 KB, while a more elaborate image might need 10 KB to look great.

Usually, I choose somewhere in the "medium" quality range for black-and-white images. Color images will often require medium or high. Note that for smaller color thumbnail images, you may need high quality.

Some examples from my Unbounded series

My urban fantasy series is heavy with images compared to many novels, a choice I might not have made if I had known more about images at the time, and it has benefited greatly from reducing the image sizes. It used to be over 4 MB on Amazon Kindle—way too large for a novel, and now it is 1.4 MB there. The cover for the first book, The Change, saved at 1400 pixels wide, is 540 KB. That's the image I use inside my epub and to upload directly on iTunes, where space really isn't a significant issue. But for Amazon, I uploaded a 1000 pixel wide cover image that is a very light 104 KB in size.

For the title page, I usually go with an 800 x 1220 pixel image, and the one in The Change is only 39 KB in size after saving for web. I've found that images with a lot of white space usually size very small with little change in quality.

In this series, I've used images for chapter headings. The example below is 354 x 135 pixels and after saving for web, it weighs in at about 7 KB. Of course, this book has 28 chapters, plus the "About the Author" section (the heading on that page is also an image), so a total of 203 KB for chapter headings.

There are also scene breaks. This image is 200 x 31 pixels and 2 KB. Pretty small.

At the end of each of my books I always include a couple sample chapters of one of my other books. I am a great believer that covers sell a book, so I include a thumbnail of the book's cover at the start of the sample. Below is a screen shot of what it looks like in a Kindle. This particular thumbnail is 300 x 457 pixels and 46 KB. With these thumbnails, I've found that I often have to save at high quality to get an image that looks good.

Final thoughts

Remember, you only need one image for all the scene breaks, not one for each, so if you're putting in multiples of the same image, stop that right now. If you're using images as chapter headings in a box set, you'll also only need one of each chapter number because you can use the same image for all the chapter ones, etc.

If you use a non-Jutoh program that may resize the images in your ebook, you need to make doubly sure to check the final file every time to make sure your quality is good. Even still, you may decide to manually resize the images to get better results. You don't want readers having a poor experience with your hard work!

Generally, even image-heavy novels like The Change should be under 2 MB, while an elaborate picture book can weigh in at around 4-6 MB for a Kindle ebook. Novels without a lot of images may often be under 1 MB, and epubs will usually be a little smaller than their Kindle (mobi) counterparts.

Creating the right image size for ebooks isn't that difficult when you follow the 100 KB per full-sized image rule. Keep in mind that every little bit of space saved on images means space you can use for sample chapters or promotional images. Again, let me stress the importance of choosing the quality for each image individually, and then checking it out on your ereader. Sometimes I had images I thought would end up large, but they often turned out to be the smallest in final size after saving for web.

Good luck! If you have any questions, please post them in the comments section below.

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