WOW! That's a real good question there! Most of the people who make models available for free, will include a Read Me which will state whether it can be used for commercial purposes or not, with or without permission. Is using a mesh in an image which wins a prize considered "commercial"? I don't know. The best I could say for right now is, if you want to do so, contact the author of the mesh and ask for that permission. Otherwise, since the contest rules don't explicitly state that one way or the other, I'd have to say that it's OK as far as this contest is concerned. I can say though that the rule was not meant to exclude imports not made by the person entering a picture. It might be a good idea to either secure permission if a model you want to use has one of those "no-commercial" clauses attached to it, or to use something that has no restrictions.