There is a bug with the detection if a cover art is present or not.
I had an mp3 file which mp3tag said it haid no cover art. Also in windows explorer (Win 7) there was no cover art visible when I had the thumbnail view. But Winamp 5 and WMP12 showed the album cover.
I tried a little test and added a different album cover with mp3tag. Now everything (Explorer, Mp3Tag, WMP..) showed the new cover. I removed it with mp3tag again, and now the old cover was still visible only in Winamp and WMP, not Explorer or mp3tag.
The cover could only be removed with Winamp.
Please fix thix, since you can never be sure if there is some crap icon the album cover if mp3tag doesn't detect it.