...this brings up a lot of interesting philosophical questions of art ownership. I know Laura is a lawyer and active APUG member who is very helpful in these matters. Hopefully she chimes in.
What if you reloaded the film and did a series of double exposures? Who would own it then? What if you got creative with the film developing and printing? My impression is that there is no ownership unless that person was to develop it because the intension of ownership is not there. If I threw out a table that I built and someone picked it up off the curb and refurbished it to sell, could I realistically come back and say "that's my table, now give me my cut?" -not likely. But I'd have to think about it long and hard to draw the line somewhere that is consistent and logical.
I've seen artists sell images they have found before and displayed them in an interesting wait and call it "their piece" in very prestigious galleries.