I discovered a pretty small and useful open source application that pulls images from Google images in a given interval and displays them as the wallpaper on the PC the application is running on. Once extracted, you can run the application right from the target location without installation; you will see a small icon in your system tray if the application is running. Right-click the icon and select options.
Add some keywords that you want to use as themes for your wallpapers, select if you want surfer friendly images only and define the change rate in minutes. You can try it out immediately by right-clicking the icon again and selecting Update Now. This takes a few seconds but the wallpaper should change into a random image that was pulled from Google. The software will only search for high quality images that fit nicely on most screens.
I suggest you add at least 5-10 keywords to receive enough images to avoid repetition. The application pulls the images only from the first page of results and picks a random image from that page which means you have a chance of 1 in 18 that the same picture will be picked again as the wallpaper.
Update: The original application that used Google to find new wallpaper images and set them on the PC afterwards is no longer available. The product homepage is gone, and downloads are no longer provided. There are however alternatives that you can make use of instead.
I suggest you take a look at the desktop wallpaper downloader and changer Wally which is available for Windows, Mac OS X and Linux. It support downloading images from Google Images and about a dozen other sources on the Internet. It too can change the desktop wallpaper automatically once it has downloaded a new image to the system.
You may want to open the options of the program to make configuration changes. Here you can change the interval in which wallpapers are changed on your system, select online and local sources, and define how those are handled. You can select Google here and all that is left to do is to enter one or multiple search terms to get started.
Right-click the system tray icon afterwards, hit play, and then wait for the first wallpaper to appear on the screen.
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