This following method should provide you with the jpegs (not sure about pngs) from your iTunes 7+ database, but you're gonna have to match 'em with the right files/directories when you're done.
Download jpegextractor from this page
. Extract it to a folder on your desktop.
Open your OS' finding app--if in XP you can hold down the Winkey and press F. You want to search your local drive for all files that end in itc
, so.. *.itc
is what you should enter. (Assuming you use iTunes 7+ these are the encrypted album artwork files and will be stored in a nested structure in your iTunes music folder.) Copy everything you find to the jpegextractor folder you made on your desktop.
Open a command window and navigate to said folder. Enter "java jpegextractor -p cover -d 3 *.itc
" without the quotes and press enter. The -p switch stands for prefix, so all of the output files will start with "cover". The -d switch is for digits--it will pad the digits at the end to 3 places (ie 001, 002, etc).