RE: thumbnails...
Hi milocool,
It depends upon the application, for instance, if you "look into" your iPhoto Library, you will find a subdirectory named Thumbnails and in this subdirectory are numerous other subdirectories that contain the thumbnails for the photos in your iPhoto Library. Same is true for video clips in iMovie Events and album artwork in iTunes Libraries.
Regards,
Switon
P.S. I would be somewhat cautious of using a mac cleaner that deletes iPhoto, iMovie, iTunes thumbnails (or alters the underlying databases) without asking you for permission.
