Bing Wallpaper is an open-source application which sets your wallpaper as Bing's image of the day, then keeps it up-to-date.
Download, unzip and run BingWallPaper.exe and it immediately updates your wallpaper, before minimising to your system tray.
Right-clicking Bing Wallpaper's system tray icon displays the image copyright message, and clicking this opens your default browser with the result of a related search. Our desktop wallpaper was a picture of a giant barrel cactus, for instance, and the search was for its species name, "Echinocactus platyacanthus", leading us to more images and information.
Leave it running and the program automatically updates your wallpaper every 24 hours. If this doesn't work for whatever reason, or the wallpaper has been replaced by something else, there's a "Forced Update" menu option to refresh the image again.
Bing Wallpaper also offers a "Save" option to conveniently save the picture as a local JPG, although it's also available at any time as a file called wallpaper.bmp in your system TEMP folder.