The ability to add tag words onto every wallpaper (specified on the wallpaper submit form). then once your wallpapers are tagged, you can use the tag cloud module in your admin to turn the feature on or off.

The tag cloud will display all the keyword tags in a big list and the more times wallpapers are tagged with that keyword, the larger it will appear in the tag cloud(font size)

Search for tag clouds if your not sure exactly how it will look