One thing that shouldn´t be forgotten in all of this chat about when a urinal is more than a urinal is that artists have never been slow to wind up the public. In my opinion, what Duchamp did then Mr Hirst is doing now, and to great success. Pablo P was also a very competent artist long before he was into his more famous distortions, for that´s all I see them to be. (Possibly just a measure of MY artistic failure, but at least I feel honest enough to admit it.)
Art, in the main, is a commercial endeavour with the artist struggling to come to terms with two monsters: his own desires and the marketplace. I have fought the same fight as a photographer and know painters doing exactly the same thing - you have to turn a buck or be born very very lucky.
But getting closer to the OP, a rip-off is always a rip-off and where the measure of any success in the new image derives mostly from the old, original subject you have stolen, then that´s all it is: theft. A passing priest may or may not turn it into something else, but that would depend on how literally you represent the old in the new; if the presence of a priest is the greater part, then perhaps you have made something - social comment for sure but art? Hmmm...
An excellent set of illustrations of this point comes to mind: in my Doisneau tome, there is a sequence of seven photographs taken from within a gallery belonging to a certain M Romi, rue de Seine, in 1947. These are from a camera set within the gallery, looking outwards to the street and including a painting of a naughty lady within the gallery window. The sequence consists of various people on the pavement and their reactions to the painting as they view it from outside. In Doisneau´s case, there is no rip-off, but a very amusing new work. But hardly art.
Taking a photograph of a car, an example quoted, is not the same thing to me because it is just an inimate object (the car) however beautiful it may be; to make the photograph mean anything you have to know how to input your own dynamic, thus taking it from the catalogue to the artistic. Also, you have to have that dynamic in the first place. This might be disputed by those who form less than natural liaisons with their cars, but perhaps we shouldn´t go there. One might apply the same negative argument to landscape: you can only get what´s already there, however much you might like to, or be able to manipulate. So is it only reportage, another found art belonging more to God than to the photographer?
But, and a big one at that, if all of this is just for fun, then more power to your collective elbow: there is little harm in finding pleasure where one may, just as long as it doesn´t mean stealing the pleasure from another.