The album art files are a part of the descriptive "id3 tag" which accompanies every audio file. You need an id3 editor to open the tag and then you can delete the artwork and make any other alterations you want to make. Personally I use iTunes to do this, I also use a Mac on which things are a bit different, so I cannot give you more helpful imformation about id3 tag editors for windows, I'm afraid.... But google is your friend
