WMP12 add album art = hidden file problem

To put it as simply as possible in Windows Media Player 12, when i try to paste album art to an album, the album gets deleted from the Media player, when i check in the actual album folder in libraries the folder itself still exists but it has changed all my files within that folder to 'hidden' so that now i can't see them or play them via WMP12. Is there a fix for this? I know a few other friends who have had the same problem and it's becoming very frustrating!

Help please :-(

Callum

What are you using to add the album art to the media files?