I just grabbed the latest version of Kindle Previewer (2.7) and a fresh copy of the Kindle Publishing Guidelines (2012.5) to see if cover image documentation had sorted out an apparent contradiction between the Guidelines and KDP Self-Publishing Help.
Nope: It’s still necessary to include an inside cover image, according to the Guidelines, and the Product Image will become the inside cover, according to KDP Help. Get the new Previewer anyway, it’s slicker and easier to use than ever.
Given that Amazon is a big place, it’s still prudent to follow all the instructions, even if different parts of Amazon appear to be butting heads like two mountain goats. Anyhow, here’s what’s in the current Guidelines and KDP Help about the cover images (italics added):
According to the latest Kindle Publishing Guidelines (2012.5):
3.2.1 Cover Image Guideline #1: Marketing Cover Image Is Mandatory
Kindle books must have a marketing cover image provided for use on the website detail page. The preferred format for the cover is a JPEG image of 2500 pixels on the longest side…
3.2.2 Cover Image Guideline #2: Internal Content Cover Image Is Mandatory
Kindle books must have an internal cover image provided for use within the book content. Provide a large cover, because Amazon quality assurance will fail the book if the cover is too small. Define covers in the OPF file…
According to KDP Help:
Entering Title Information: Product Image (optional)
Upload a cover image here. Cover images can help entice readers to take a closer look at your eBook. Images must be in TIFF (.tif/.tiff) or JPEG (.jpeg/.jpg) format. Please size your cover art at least 1000 pixels on the longest side. Note: This image will be embedded as the cover inside your book and will appear on the product detail page and in Amazon search results.
Formatting Your Book: Creating a Catalog/Cover Image
Please note: when you publish or re-publish your book, your cover image will be automatically added to the inside.
For best quality, your image would be 1563 pixels on the shortest side and 2500 pixels on the longest side
In the Guidelines, we are told to include a large inside cover but large is not defined. On eInk Kindles, a 600×800 pixel inside cover image fills the screen perfectly. On Kindle Fire, the full screen viewing area has different proportions, taller and thinner at 150x89mm, almost matching the 1.6 aspect ratio of the product image cover.
Since the inside cover is replaced by the product image cover, it doesn’t hurt to stick with 600×800 pixels (smaller upload) but going forward, I would be inclined to use an optimized image with the same 1.6 aspect ratio as the product catalog cover. That is, the cover will be 60% taller than it is wide.
Right now, it appears that Amazon resizes the product cover image for each Kindle device or eReader app used for viewing. I don’t know whether this happens on the fly or if several resized images are stored in the final .azw file. Either practice would be a practical solution for multiple device/eReader compatibility if image quality were better on the inside covers. That is, inside covers currently suffer from too much compression going from the very large catalog cover down to Kindle sizes. Looking ahead, this problem may be self-limiting in that bigger images will be needed for higher resolution devices that are becoming the norm with the iPad Retina Display, Kindle HD, and Kindle Fire.
The best thing about the new cover image size (recommended) dimensions is that Zoom works great for the cover when Look Inside the Book is available. That’s an awesome feature for promoting book sales and restores the functionality that was lost with the older product image Zoom and Pan feature.
Note that there are no explicit instructions on the KDP Formatting Help page for the inside cover versus inside images (illustrations). They aren’t the same. Inside illustrations fit inside screen margins and above the formatting bar; the inside cover fills the screen. Full-screen illustrations work best at 520×622 pixels on eInk Kindles. For more information see Kindle Maximum Image Size.