Uranium Desktop lets you automatically rotate your wallpapers in 2 easy steps: First you specify one or more folders where your wallpapers are located. Second, you set the time interval, ranging from every few minutes to every month. You can have individual wallpapers on each monitor (or one image cloned or stretched on all the monitors). Features smart wallpaper display: wallpapers are proportionally stretched to fit your screen, background color is automatically extracted from the image (if possible) and small images are automatically tiled.
