Does it have to be a new image or can it be something from our gallery that seems to fit the subject?
Being that the artist will be known anyhow if we are posting the entries to the forum, I dont see any reason why it should be new images only. Might boost participation if we dont have to drop what we're doing to create a special image for the challenge.

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/