MM5 5.0.0.2127 (same behaviour in 2126)
Album Art embedded as "Cover Front" in ID3
Upload a song to GPM with MM5: no album art in GPM afterwards
Upload a song from the same album to GPM via browser: Album Art is shown
Another difference: Total track number is seen.
Hint: If I upload many songs through MM5 a small percentage (estimated less than 10%) is showing Album Art in GPM. Unfortunately, I couldn't find any obvious difference in the ID3 tags.