Thanks for that info, Turtle, and also your suggestion, JSWolf.
I've done a bunch of experimenting, and have gotten the best results by converting the ePubs to AZW3. My primary test file was an ePub with a cover image that was 322 KB. The image doesn't appear to have been downgraded at all in conversion to AZW3. I stress the word appear, as I cannot confirm what is (or isn't) happening behind the scenes during conversion. The results speak for themselves, though. I cannot discern a difference between the cover in the AZW3 file and the original image, except for it being displayed in black-and-white. This was not the case when I converted the ePub to Mobi; the Mobi image was very obviously downgraded.
I'm going to proceed with converting everything to AZW3 without tweaking the original images in the ePub files. I'm relieved that I won't have to bother with that latter step -- I have a lot of books with illustrations!
Last edited by Skibble; 02-05-2013 at 03:31 PM.