Hi Jim
I appreciate your interest in finding a solution. Here's a rundown of the current status quo:
Rockbox only supports BMPs (may change one day). The pixel size depends on the theme (skin). Official builds don't support on-the-fly resizing during playback (yet), though unofficial builds do. Here's the Rockbox Wiki page with more info:
http://www.rockbox.org/twiki/bin/view/Main/AlbumArtYou may notice the list of links to utilities listed at the bottom of the above page for converting formats and size. I've checked them all out and not managed to find any way to make them work efficiently with MC/Rockbox
Perfect would be a MC on-the-fly conversion of cover art during handheld sync that converts the format to BMP, changes the pixel size (currently needs to be user/Rb theme specific), renames it to something supported by Rb and then stores the BMP appropriately for a sync so it lands somewhere that Rb will find it. Here are the options:
./ filename .bmp - same filename as currently playing music file
./ albumtitle .bmp - name of the album, found in metadata of the music file
./cover.bmp
/.rockbox/albumart/ artist-albumtitle .bmp
../ albumtitle .bmp
../cover.bmp
In fact, the utility would be a very similar function to the current sync file conversion for audio formats.
Let me know if anything's not clear.