Thank you jainen, I understand what you mean, of course the point is to pay the correct taxes, this is what we are going to do. Maybe I didn't explain myself clear in the first post and that's what generate the confussion. We really know the client is not in a photography business, we do know this was a prize for a one time contest, and photography is not even a hobby for him, so we wondered why they reported the prize in box 7. Knowing this, we try to figure out which would be the best option, line 21 of form 1040 (explaining that it was a prize on that line) or fill out schedule C, not because the taxes to pay are different (they would be the same), but because we wanted to discuss with the users which option gives less problems.
Pacun, I don't think I said anything to be offended by you, not all of us have the same knowledge as you seem to have in this matter.