To be clear, there is still technically a limit for image file size, but it has been (or will be) increased to 5MB per image, regardless of booktype. This effectively removes any restriction on image size, since virtually any image file can fit within this limit easily, even at high resolution, and adding images of even this size will increase the overall file size dramatically.
But more on that in a minute.
First, I have highlighted and annotated the changes in my copy of the Guidelines, which you can download here. Or just get a clean version from the link above, since the first change you'll notice is the addition of a Revision History:
This is a seriously welcome addition that makes the task of finding out just what exactly has been altered from one edition to the next far easier. Still, for your benefit, I will share with you here some of the details.
188.8.131.52 KindleGen Messages
This section received some editorial clarification to distinguish between Errors and Warnings in the KindleGen message logs, and their respective results. Nothing technically has changed, but it is made much clearer that Errors result in an abort of the conversion, while KindleGen will attempt to fix issues with Warnings, but this may or may not work, or work to satisfaction (for which Amazon declaims all responsibility).
3.2.1 Cover Image Guideline #1: Marketing Cover Image Is Mandatory
The recommendation for the "marketing" cover image size (the one you upload to KDP) is now 2560 x 1600 pixels, with 350 dpi resolution "to insure image clarity on Kindle HDX devices." This is altered from the last edition from a 2500 pixel recommendation for the longest side, with a minimum of 1000 pixels, but no specs given for the short side. An additional note is given stating that you will now receive a "reminder message" during upload if this image is smaller than the recommended size. The absolute minimum is still 500 pixels on the smallest side, so the warning can be safely (though not wisely) ignored if displayed.
Incidentally, the dpi resolution is essentially irrelevant on digital displays, since it is the total number of pixels that determines what is displayed onscreen. Amazon has previously always recommended 300 ppi, but with the Fire HDX 8.9" packing in 339 ppi they apparently felt obligated to add the higher dpi value, even though it doesn't matter in the least, especially since the marketing image is only used for the book's web page.
Lastly, here is where we are presented with the first indication that image file size has been given some consideration by the powers that be behind the scenes, as it states that "the image file size should be 5MB or smaller," which is a new addition.
3.6.2 Image Guideline #2: KindleGen Performs Automatic Image Conversions
Now we get to the crux of the matter. The entire section relating the various image files size allowances for the various booktypes (i.e. 127, 256, or 800 KB for flowing, childrens, and comics, respectively) has been removed. In its place we get this:
"The maximum size of an individual image file is 5 MB. The maximum size of an epub is 650 MB."Moreover, while the header to this section still references "Automatic Image Conversions", the portion that formerly detailed the manner in which KindleGen handles "quality factor reduction" (i.e. image compression) has been removed. Apparently this will no longer be the case.
Now, with that said, bear in mind that the current version of KindleGen is still the 2.9 build that was released last September, so we will need to wait until the (presumably impending) release of the newest iteration (2.91? 3.0?) for any of this to take place. In addition, the posted Release Notes, as well as the relevant KDP Help section, still list the older version and lower size limits (although the KDP Help has never even been updated from the original 127 KB image limit for all Kindle ebooks, so I wouldn't put much stock in it as far as accuracy is concerned).
At any rate, we will presumably see an update to the Kindle Publishing Tools quite soon, if these changes are any indication. A few other entries add additional support to this assertion, as well as offering a further update in Amazon's outlook on image size:
4.3.3 Recommendation #3: Optimizing Content for Full Screen
Here (as well as Section 5) the reference to the original Kindle Fire's 1024x600 resolution has been altered to the newer Kindle Fire HD 8.9" display's 1920x1200 pixel depth. This not only increases the recommended image size, but alters its aspect ratio from the prior 17:10 to the newer HD models' 16:10 ratio, now apparently the preferred format (and a step in the right direction, although I'm still a strong advocate of 4:3 on e-readers for the sake of two-page spreads in illustrated works).
Additionally, the previously laughable statement that in order to "support 2X magnification with high quality" in children's books, "image pixel dimensions should be at least 2048x1200" - an utterly unwieldy resolution for a 256 KB file if quality is a concern - has now been increased even further to a recommended 3820x2400! There is no way in this world or any other that an image that size will look even reasonably decent at less than 256 KB.
5.2 Asset Requirements
This is the section that gives the breakdown for "Zoom Factor" values for region magnification, which again have been increased to accommodate the HD displays:
100% - 1920 x 1200Curiously, this section still states that images must be smaller than 800 KB in size, but I chalk that up to Amazon's standard sloppy editorial practices, since this sort of thing has happened before (and often) throughout the various editions over the years.
125% - 2400 x 1500
150% - 2880 x 1800
250% - 4800 x 3000 [!!!]
Again, these "recommended" image sizes are simply unattainable at the previous (or rather, still current) file size limits. 4800 x 3000 is simply comic in this respect. But at 5 MB I can give my readers glistening crisp and brilliant detail even when zoomed to the highest value. Overall file size, of course, will still be an issue, which brings up a point I sort of glossed over in the earlier section, and that is the mention of a 650 MB file size limit for epubs.
This could possibly mean that the heretofore consistent KDP portal limit of 50 MB for file uploads might be increased (although I doubt it). Since this is not a well documented limit, only a test will determine the truth, and I haven't done one yet. The KDP Help page referenced above still lists the 50 MB limit (which is, as far as I know, the only place it's actually given), but again, it still lists 127 KB as the image size limit too, so don't put too much faith in that.
However, we should remember that even though flowable Kindle files have had the ability to contain audio/video files of up to 600MB for some time now, the portal limit has not changed because of it.
What this really refers to is the maximum input file size for KindleGen itself, as mentioned in Section 6.13 on audio/video file size, where it explicitly states that "the total maximum audio/video file size that can be converted from EPUB via KindleGen is 650 MB." This apparently now applies to fixed layouts in general now as well, and not just audio/video content.
3.6.11 Image Guidelines #11: Use Supported SVG Tags and Elements
A few other sections should be mentioned, of which this one is entirely new. This two page entry provides a list of Supported SVG Elements, along with an example and notes on tag usage. Also included is a link to the SVG specification for reference.
3.12 External Link Guidelines
And speaking of links, this is another new section laying down some laws regarding proper hyperlink behavior. Most are standard "no offensive content" warnings, but it is also made explicit that links to other online retailers are forbidden, and that Amazon "reserves the right to remove links in its sole discretion," a significant stipulation in legal terms.
There are no functional changes to this section, but I mention it because it contains what appear to be the very first indications that Amazon has actually acquired an editor, since all of the several alterations here are purely for the sake of phrasing. Changes, for example, from "quickly search" to "search quickly" or "the entry they want" to "the desired entry" are entirely aesthetic in nature, and bear no substantive difference.
That said, it is made more clear that dictionary functionality is for "in-book search and lookup" and that they must be marked as such, and with the correct language tag(s) applied, for what that's worth.
11.1 Appendix A: HTML Tags... / 11.2: Appendix B: CSS Selectors...
Although these are each listed in the Revision History, I cannot see any changes made here. All the "No's" are still "No" (i.e. no audio/video support in KF8; no max-width/height attributes, etc.) and there are no additions or deletions that I can discern. If anybody spots them, let me know!
* * *
With the release of the HDX displays and resolutions reaching the limits of human perception, the Kindle format is long past due for a major update in this respect, and I can't express how pleased I am to see the changes to the Guidelines. I have been waiting as patiently as possible for technology to reach the point where e-readers can accurately replicate the full size print experience in pristine image quality, since it is clear that digital is the future of the reading experience, or at least a very large part of it. Unfortunately, illustrated content has been sadly left behind in this regard (aside from iBooks on the iPad, which is superior as a graphic format, if not in terms of sales and store support).
With this update Amazon's Kindle platform will once again have a chance to truly shine where visual ebooks are concerned.