So what you're saying is you've got two programs... Musicmatch, and some unnamed tagger.
Both have options to add pictures? Musicmatch works and the other one doesn't?
My guess is this:
Each of those two programs adds the pictures in a different way obviously, and I'm guessing that one of them isn't even using a proper ID3V2 tag, it's just appending the image to the end of the file. I've seen that sort of thing before, where a program just adds nonstandard crap at the end of an MP3 file and expects playback software to sort it out.
The trick is figuring out which one is which, and more importantly, which of the two that your player is expecting to display.