"For those who like to keep their wallpaper out of the main
system settings, or just have a lot of images to choose from,
Wallpaper Tray's an attractive option. The app, which you manually
add to your auto-starting apps, can change wallpaper on a timer and
set scaling like its brethren, but also lets you maintain multiple
folders of wallpapers, scanned at each startup. You can switch
backgrounds from the tray icon, and there's even a search tool for
finding the specific image you want to change to..."