I think that you cannot use an already made image for the challenge. If you post your image as a challenge entry and also in the gallery then I think its ok that way. They just don't want people saying oh I made an image 6 months ago that fits the challenge category. Challenges are just for fun and learning and can also be put up in the gallery before voting, at least I think that I have seen that done. In some cases people have linked to a higher Res gallery image as their entry. HTH (and hope I'm not wrong) :-D