Thank you for all of those excellent tips! There was also a lot of good information in your other post you linked to. Very helpful.
About half the images are photos (jpgs) from a variety of sources, so there is no one resolution, but I purposely sought out the highest quality images I could find for the subject needed. Using PS to change PPI to 300... most were 240PPI so not a big jump. Also making them a uniform 600px wide. And again, most were close to that. Then Save for Web to compress/strip metadata.
The other half include transparency (just about all created in PS), so made those pngs.
There is a nice little (free) plugin for Photoshop called SuperPNG. It does a great job of compressing/removing metadata.
Having tried a few compression/meta-stripping programs prior, I found the two methods above to give the best results, in my case, for my images.
BTW, Amazon says not to compress the cover. Mine is the Amazon-recommended 1600x2560 300PPI (EDIT:
I saw they actually say 72DPI is fine, and slight compression too).