This has happenned to me on a few occasions. In my case it was not the intention of the offending party to "pirate" my photos but simply ignornace that what they were doing clearly violated ancestry.com rules. Sometimes people download a photo to their hard drive for their own files and then upload it from there. They may try to improve it themselves to re-name it or what not. This has happenned only a few times to me. In the vast majority of cases people simply link the photos from my tree to theirs, which is what ancestry intends. This way a user can always trace back where the photo originated. I'm fine with that.
In cases where someone either ignorantly or maliciously uploads one of your photos I would simply leave a comment on the photo in their tree. Something to the effect of "this photo was originally uploaded by.... from ...... tree. See ..... tree to see the original. If the tree owner removes the comment you can report them to ancestry.com for copyright infringement. If they did it out of ignornance they may just apologize.
This irritated me when it happenned at first, but you do have to take the good with the bad. Don't let a few bad apples spoil your ancestry.com experience.