Webilder is a greate open-source program that automatically download new photos from Flickr and Webshots.You can keep getting most interesting photos and automatically change your wallpaper every few minutes.
To install webilder,click here to download deb package,or read this page for more.
After install this,you can manually download new photos or set to automatically download every few days.And preview these windows in this window:
Some of these photos: