Mine changes constantly. I have a screensaver program from Second Nature Software On-Line Store Home Page
, which publishes sets of high-quality nature photographs (and some others). A significant portion of their profits goes to the Nature Conservancy
, an environmental group I support. I have several of Second Nature's photo sets that I've purchased over the years. The last photo displayed by the screensaver becomes my new desktop.
You can try their screensaver with several sample photos for free. Just go to the site (link above) and click on "free screensaver and updates" in the upper left of the page.
Here's my current desktop (which will change once I stop using the computer for a few minutes):