In Linux, the following will list directories under "/your_mp3_dir" that don't contain "cover.jpg"
find /your_mp3_dir -type d -exec test ! -f {}/cover.jpg \; -print
If you wanted to copy a file "standard_cover.jpg" into each
find /your_mp3_dir -type d -exec test ! -f {}/cover.jpg \; -exec cp -n standard_cover.jpg {}/cover.jpg \;
Or if you'd like to save some space (and all the files are on the same extN partition);
find /your_mp3_dir -type d -exec test ! -f {}/cover.jpg \; -exec ln standard_cover.jpg {}/cover.jpg \;