I usually put up photos that I have taken. I took this picture of a mountain lion yesterday at the MN zoo and thought it turned out pretty nice. I don't usually keep a background up longer than a week and it is ALWAYS nature-oriented.
Here is my desktop wallpaper... (I took the photo on my cruise, then added the text to it). I've done this with about 10 of my photos, and also made a screensaver out of one whole roll of film.