You should be able to click and drag into the window on the lower left when an album is chosen. The only exception is with WAV files, which, last time I checked, did not support the drag-and-drop method. There is a separate folder in your itunes folder:
Music>iTunes>Album Artwork
But I'm not sure if it's wise to just delete that to start again as I don't know how the file structure works there.
Music>iTunes>Album Artwork
But I'm not sure if it's wise to just delete that to start again as I don't know how the file structure works there.