A photo of people (family, friends, myself, ...)
A photo of my pet(s)
A photo of something man-made (cars, machines, etc.)
A photo of nature, space, or something else not man-made
An abstract pattern
Artwork, digital or photographed
A solid color
A digital animation
Whatever my desktop environment has set for me by default
I haven't seen my background in weeks, too many windows covering it!