While the image size limit is now stated to be 5 Mbper image, rather than the previous 127 kb for reflowable files and 256 or 800 kb for children's and comic fixed layouts respectively, there has been no official announcement from Amazon on the matter. Moreover, there has been no release of an update to Kindlegen that might incorporate this change, and files converted via Kindlegen or Previewer still compress the images to the previous levels, as revealed by extracting the contents using KindleUnpack.
How, then, is it possible for the higher 5 Mb image limit listed in the Guidelines to be true? Is this just a future upgrade waiting to be made? A simple typographic error? Or is there some unwritten information hidden between the lines?
In an email response to my query on the matter received this week, an Amazon rep said:
From Version 2.9 of Kindlegen, images up to 5MB are retained for High Definition devices. For Standard Definition devices, however, the images will be compressed.The answer, then, is different images for different devices! In fact, Amazon has apparently been doing this for some time already without telling anyone. Since the converted mobi file contains both the compressed images and an untouched source file zip archive, Amazon is able to send alternate versions of each ebook to different devices, depending on their resolution.
Of course, I wanted to know for certain if, and how, this worked in practice, so I did some tests this morning to determine if it was, in fact, the case. To do this I created a fixed layout, comic book-type test file with 6 pages, each containing just one full page image, ranging in file size from 1.8 to 4.94 megabytes. This created a source file of 22.2 Mb in size, which Previewer converted into a mobi file of 48.6 megs. I also uploaded the source file to KDP, which resulted in a download preview file of an only slightly smaller 45.5 Mb. In addition, I tested variations with the children's book-type, and as a reflowable file, all of which resulted in visible artifacting in the compressed images when extracted. However, when these same files were side-loaded to my HD 8.9" Kindle, every image looked crystal clear, showing that the reading system was accessing not the compressed images, but the high definition source files instead!
Here is a screen-cap comparison of the high quality image file being rendered on the HD 8.9 (left) and the compression employed on the same image for the standard definition file (on the right):
The HD device is clearly accessing the uncompressed image from the source archive file, rather than the highly compressed version found in the mobi8 image folder. Now, bear in mind that this is a 127 kb image that has been compressed down from one nearly 5 mb in size, so an incredible amount of compression has occurred. Moreover, Kindlegen will shrink the actual resolution of the image in reflowable files and fixed layouts with the children's book-type, although not in ones with the comic book-type, so this comparison is between the two extremes and probably not a common occurrence. But you should know that this can happen, and is, in fact, happening to your ebooks already.
One more important note must be mentioned here with regard to including very high resolution images, which is that it severely bogged down the device response, causing delays in page turns, sluggish response to scrolling and zooming, and in general behaving in a most unpleasant way, much like the old days of eInk displays (oh, wait, they still have those, don't they!). What this means in practical terms is that, although you can now include very large images in your Kindle ebooks, you will want to carefully manage the ultimate file size at which each image can be delivered, since you can no longer assume Amazon will do this for you. If you include 5 Mb image files, they will now send those full size images to the end user's HD device, for good or ill.
Which brings me to my final point, and that concerns Amazon's delivery fees. One major concern with image file size increasing has been the prohibitive cost of a .15 cent per megabyte bandwidth charge incurred under the 70% royalty option, since a single 5 Mb image would therefore cost .75 cents to send! But surprisingly, this does not prove to be the case.
While the KDP preview file I downloaded today weighed in a 45.5 Mb, the "book size after conversion" listed on the product pricing page was only 3.36 Mb - the size Previewer's log listed as the "deliverable file size" after conversion. This makes the total download fee just .50 cents for the full size HD ebook, leaving $1.74 profit for a $2.99 list price title. Amazon has essentially decided to subsidize content for the HD devices by only charging for the lower file size. This makes perfect sense, since you cannot rightly charge the higher fee for files sent to lower resolution devices, and charging different rates for different devices is simply impractical (not to mention a bookkeeping nightmare). It's also probably why they haven't bothered to publicize this much. Or at all.