Sleex Wallpaper Changer (free)

Sleex is a simple wallpaper changing application based on the Google Image Search. It changes your wallpaper periodically at the given rate allowing you to configure the search phrase (the same as you search for images on Google), safe search feature (filters out porn and similar), aspect ratio.
The paid version can additionally save all downloaded images to the SD card, keep the history of downloaded images decreasing the chance of repeating, update the wallpaper while the screen is locked, start the service after device start.

Application requires the internet connection to work.