There are a number of threads on the message boards on this general topic. Here is one that may be of interest:
http://boards.ancestry.com/topics.ancestry.ancsite/11788.2.1...If a photo is attached to the wrong person, and the person who attached it will not remove it, you can add a comment on their copy (which they can't remove unless they delete the photo itself). This will be seen by everyone who has copied it (correctly or otherwise)
Just add something that says something like, "This is my grandfather, John Edward Smith. I uploaded this to my tree from a family photo album. This man was born on X date and died on Y date. He was married to Eliza Jones. He lived all his life in Ralls County, MO. This is not the same man as John Aaron Smith, born on A date and died on B date, who was married to Susannah White and lived in Berkshire County, PA." In future, you might want to put a watermark, e.g., with the person's dates and possibly location, on the photo before uploading.