Thumbnails are reduced-size versions of pictures, serving the same role for images as a normal text index does for words.
Thumbs.db no longer exists in Vista. This data has been moved to \Users\\AppData\Local\Microsoft\Windows\Explorer
KDE & GNOME
Example thumbnail in GNOME:
$ hachoir-metadata .thumbnails/normal/0d97afdc637ac86d75d13e72172dc77c.png Metadata: - Image width: 128 pixels - Image height: 122 pixels - Bits/pixel: 24 - Pixel format: RGB - Compression rate: 1.6x - Compression: deflate - Producer: GNOME::ThumbnailFactory - Comment: Thumb::Image::Width=779 - Comment: Thumb::Image::Height=744 - Comment: Thumb::URI=file:///media/truecrypt1/123.jpg - Comment: Thumb::MTime=1216153400 - MIME type: image/png - Endian: Big endian
GNOME will save thumbnails for files on mounted encrypted filesystems.