I think I usually use a different term - desktop background - for what you are referring to.
In any event, I usually use something I've shot myself. It is a good way of getting a sense of how well a photo will stand up to:
a) repeated viewing; and
b) numerous distractions.
Naturally, the choice of photographs is somewhat constrained - I don't like using something that makes it difficult to find something on my desktop.
I've had the attached as my desktop background for a little while now.