You don't have to own the original image. You have to photochop the original image so much that it becomes a new piece of work. To quote "A work must pass a threshold of originality in order to be copyrightable." - What it means by be copyrightable is that it is a new work and is then owned by the "photochopper".
The question is, at what point and how much photochopping passes the threshold. Which there is no actual clear line in written law.
Here is a good wikipedia page on "Threshold of Originality" -
http://en.wikipedia.org/wiki/Threshold_of_originality