if the covers are all blank boxes then its most likely the out of memory issue caused by having too many games and the images of your modified theme being too large in bytes.
@blackb0x shrunk (can't think of right term) all of rhapsodii shima images to make them smaller in bytes but still look good.
what happens is the cover is downloaded and saved to usb or sd card but the making to wfc part doesn't work do to not enough free memory. even though it says it makes it - it doesn't. @Wiimpathy created a homebrew app that does the converting to wfc that you may want to try. its called CreateCache - https://github.com/Wiimpathy/CreateCache
You are awesome @fledge68 !
It worked like a charm!