My thought is this:
1) It has to be the owner's car, but not the owner's pic. In other words, the owner can have someone else do the shooting.
2) The photoshop thing is always going to be a bugaboo. Almost any shot need sto be adjusted. What constitutes too much adjustment? My best suggestion is to say people can adjust brightness, contrast and saturation. That should eliminate what I think is the purpose of a rule like this, things like photoshopping one's car onto an aircraft carrier deck or something like that. (BTW, I don't see any problem with photoshopping the filler shots.)
3) One shot per person: was this meant to be one shot per car? A few people submitted multiple shots, but not more than one of each car. Do we want to limit owners to submitting shots of several cars? (For that matter, what if someone submitted a shot with several cars in it?)