I know a few people were finding that they had some missing album art when using a USB stick. I too had this problem although the tracks were showing album art just fine on a PC. I solved the problem by using the program dbpoweramp and its ID Tag plugin to apply the manipulation 'force embedded album art to jpg'. I ran this over my whole music collection and now all my albums show with album art on the touchscreen. I am guessing that the ID Tag plugin applies the art to the files in precisely the correct way whereas some other programs do not.
Hope this helps.
CJ
Hope this helps.
CJ