It's just because your thumbnails aren't updated with your new settings, that's all... (it isn't because they are hidden, your display settings are wrong, or whatever)
Whenever you change the thumbnail settings (thumbnail size for example) the thumbnails wont update automaticly.
The same goes for when you alter the quality of the thumbnails, if you reduce/enhance the quality you need to update all your existing thumbnails....
The updating can be done very simply: (don't alter your display preferences, color settings and such things, it doesn't have anything to do with that
)
* Delete
Thumbs.db that you'll find in that directory (it may be a hidden file), after that refresh the view...
* Or, select every file/thumbnail in the directory (CTRL+A), then right-click on a thumbnail and choose
update thumbnails....
/ 
