I concocted a scheme where I would subscribe to flickr feeds with enclosures and use the folders that it creates to seed the Microsoft Power Toy Wallpaper Changer with those photos for a fun, constantly changing wallpaper. Clearly I spent too much time thinking about it, and not enough time searching for someone else's application that already does this for me:

John's Background Switcher periodically changes the background image on your computer (like every hour or every day).

[ John's Adventures: John's Background Switcher ]

Integrated right in the application is:

You can use Flickr (almost certainly the best online photo management and sharing application in the world) - you can choose to select pictures by person, tags, sets or just plain random and there are a host of options to narrow down the pictures and increase the quality of those chosen. You never know what you're going to get next!

I recommend the Flickr Group Domokun Lovers.