Recently I needed to extract the album art from an MP3 file and came across a really easy to use command line utility called eyeD3 to do just that (among other things). Here is how you can extract all of the album art from a file MyFile.mp3 into a directory called Output.
1) Install eyeD3
sudo apt-get install eyeD3
2) Extract all embedded album art from the file
eyeD3 --write-images=Output/ MyFile.mp3