There are issues with some applications messing with live tiles. Two culprits are Sage Thumbs and FastPictureViewer. If you don't have those installed then I'd look at applications that alter or do anything with thumbnails or any image applications especially ones that mess with or add codecs. These will be the more likely suspects.
I've also heard that if thumbnails for .PNG images aren't showing up correctly in explorer that it is linked to this issue. Fixing whatever it is that is causing the issue with .PNG thumbnail, fixes this too.