In my experience, it's because the music folder is setup as a 'folder' in Windows Media Player (read again - WMP... not Windows Media Center). So, I recommend going into WMP, go to the pictures library, and remove all the folders in that library that you do not want media center scanning for your pictures area.
Same thing for videos.
Jon
hondabuf:why am i seen my music album art in the photos section of Media Center?