Most versions of Windows do a pretty good job of changing your wallpaper, whether once a day or every few seconds. But if your version of Windows lacks those features (some do) or you're running an older version of Windows (some do) or you just don't like the Windows tool (some don't), you can use aftermarket freeware like 100dof Wallpaper Rotator.
It's a simple tool that can rotate your wallpaper at startup or manually. It stretches images to fit, and it handles the most common image types. What sets it apart from the crowd is its ability to recognize shortcuts and use them to access images in different locations. You can create a wallpaper folder containing images as well as shortcuts to images and use Wallpaper Rotator to access them all with one interface.
Wallpaper Rotator's installer lets you select the option to rotate your wallpaper at startup. This is important because you can't change this setting without uninstalling and reinstalling the software. We also had trouble with this feature in a 64-bit Windows 7 installation, which ran Wallpaper Rotator in Compatibility Mode. We repeatedly received corrupt JPEG warnings, whether we chose images on our C drive or a USB drive. But the program functioned normally in a 32-bit installation, and it also worked well in 64-bit Windows when we deselected the startup option.
We browsed to a folder of images, including subfolders, and chose our rotation and fit options. We selected Stretch to try one of Wallpaper Rotator's other interesting twists, the ability to stretch images from the inside or outside, with a check box to keep the image ratio. By selecting the image ratio check box and inside stretch, we were able to fit a wide range of image sizes and shapes to our desktop. A color picker let us change the background color, too.
Wallpaper Rotator does a nice job in compatible systems. We'd like to see the ability to configure the program's startup behavior from the options panel, plus some timing options that could make it a more viable option given what Windows already offers, and more reliability.