Attached are the originals and the associated Kindle screenshots for 3 jpg images. The first is a cover image from FeedBooks, its original size is 595x842 (37KB) and it gets rescaled just like images in the interior of the ebook. The last two are from bob_ninja's version of
Potter, Beatrix: The Tale of Samuel Whiskers. v1, 16 May 2008, one is 931x1264 (117KB) and the other, in color, is 603x736 (74KB). All three images are relatively tall but this confirms the 520x640 actual size, the rescaling (maintaining aspect ratio) and the support for jpg's > 64KB. There is a noticeable pause before displaying a page containing an image, but illustrated ebooks work well on the Kindle. It is a shame that some professionally produced ebooks have much smaller images than those shown here.