Album artwork

Hi

This isn't really a "problem" as such, I'm asking out of curiosity.

I normally add album artwork by using Windows Media Player's "Paste album art" option, which inserts a 200 x 200 graphic and in MP3tag this artwork shows as "front Cover". Most of the time it also says "thumbnail" but sometimes it doesn't. Does anyone know what causes this difference in behaviour?

Thanks in advance for any info about this.

Cheers

Baz