Hi! I messed up with covers and now I can't reset to original state:
I tried clearing albumcovers folders and using unset cover from cover manager - covers are cleared, however some of them now appear as blank (nothing, transparent image), instead of default picture for cover. Does amarok store some aditional info in DB (I'm using mysql)?
If so, what should I enter in mysql console to clear such info?

Clearing the DB did the thing. Amarok cleared DB automatically when switching on/off last.fm services (bug), reported to bugs.kde.org, bug # 126890.