Album Art - Shows up on iPhone 4 but not Mac

I study language vocabulary using a large collection of m4a files. Each file contains the audio of a word in one langugae with corresponding album art and its meaning in another language with corresponding album art. Thus, there are two images associated with/embeded in each m4a.

On my Mac, I can see the album art work if I click "Get Info" and then look under the "Summary" tab. However, when the I play each file, I can see only the default music note image. But, on my iPhone 4, all images display correctly.

These files used to work correctly. What happened?