Why are image sizes inside eBook difficult?
The "story" of image sizes inside ebooks is a long one. The bottom line is that the advent of the newer, high-resolution devices has made image embedding a much more complex job. There used to be an uber-handy Wikipedia article on the topic, where you could look up all the screensizes, resolutions, etc., by device, but some overzealous, politically-correct Content Nazi decided that this usage was "too commercial," and removed the article. (As you can tell, this just irritates me no end! If someone at Wikipedia grows a brain, and reinstates the article, I'll revive the struckout content.). You can see the screen resolution of all the Kindle devices here, in this Wikipedia article: http://bit.ly/1qrmIgT. The range of screen sizes and density is amazing. (If your question is about COVER sizes, please see this FAQ article, What is the Correct Cover Size for Amazon eBooks?, instead. This article is about interior images inside of ebooks). Although we talk primarily about Amazon devices here, the same issues and challenges are true for the Nook HD devices, as well as the later iPads versus the first-gen iPads, and the like.
Resolution of Amazon's HD Displays
But what matters to you is this: each device displays pixel-for-pixel. What does that mean? Let's assume that you're doing your Mom's memoir. And in the frontispiece, you want to put a lovely picture you have of her. The image that you give to us is, for this example's sake, 424 pixels wide. In your Word file, it was 5.88" wide, taking up nearly 3/4ths of the width of your original manuscript page. You scanned that image of Mom, on your home scanner, from an old photo, saving it at 72ppi/dpi. How big will that ebook image be, when it's viewed on a Kindle? The answer is: it depends.
- On a Kindle Paperwhite and Kindle Fire HD 7", it will be about 2" wide, across the screen.
- On the Kindle, Kindle Keyboard 3G, Touch 3G, Touch, original Kindle Fire and the Kindle DX, it will range in size from 2.85" in width to 2.5" wide.
- On the Kindle Fire HD 8.9", it will display at 1.66" wide--about 1 and two-thirds inches wide.
- On the Kindle Fire HDX 7", though, Mom will only display at 1.31 inches wide.
- And on the big HDX, she'll be barely 1.25" wide. Basically, the length of your thumb, from the tip to just past the last joint (nearest the tip). Not even a quarter of the screen width. That's a bit small for good old Mom, isn't it?
And what about height of images? This gets to be even greater fun.
- For an image to be full-height on the older Kindles, the old "800px tall" standby works pretty well. That only works for the old devices, however. The heights for the others are far, far greater.
- The Paperwhites are 1024px tall. So is the original Fire.
- The DX is 1200 pixels tall.
- The Fire HDX 7" and Fire HD 8.9" are both 1920 pixels tall. If you put an 800px tall image on those, it would only cover 40% of the screen height.
- And the big HDX, 8.9"? That's a whopping 2,560 pixels tall. An 800px tall image on that would be lost, covering merely 1/3rd of the page, in terms of height.
So, what do we recommend?
Yes, this stuff gets confusing. In terms of image-to-screen coverages, our best recommendations at this time are to use 1200px for height, 600 pix for half-height and 300pix for quarter height, all saved at a minimum of 300ppi/dpi. If we receive these images at a high-enough resolution (use 300ppi/dpi in your Photoshop, Gimp, PC Paint, etc., program), we can ensure that these have enough clarity and crispness so that even on the larger screens, they'll produce clearly and with good detail. If you send us images that are 72ppi in resolution, the likely outcome is going to be that it's going to be quite a bit smaller than you expect on the higher-resolution devices. Always output your images, or save the stock images you're using, at 300ppi, to ensure that we have enough "pixels" to work with, and to prevent you from having any unhappy surprises.
You will probably read, in various places, ebook "experts" who will tell you that they use smaller images, and "just set it to be 100%, so it always fills the screen." While that sounds nice and easy, and almost sounds like it makes sense, if you've ever seen an image blown up too much, that's what can happen if a smaller image is simply enlarged too much--it becomes pixilated. So you want to be sure, whenever you can, to use or give us the best-possible image quality for your books. We use custom coding for each and every image, to ensure that it is displayed at the best possible size for every device.