I think it depends on the device you're using to read Kindle books, and the version of the reader software/app you're using. With Windows and Kindle for PC, the versions of K4PC that download KFX format will download more files and one of them is the azw.res file with some higher quality images.
There are some other threads about it that go into more detail.
https://www.mobileread.com/forums/sh...d.php?t=295611