I totally agree with the frustration, and when I was seeing my own personal photos coming up as hints on my tree, I contacted ancestry. After a lot of discussion, and probably complaints from others, it was discovered that some of the problem is a result of the sync feature of Family Tree Maker. When a tree is sync'ed from ancestry to a personal FTM, it does not maintain the originators information. Then, when it is sync'ed back up to ancestry, it appears as if that tree owner was the originator. Ancestry said this will be fixed in future releases of FTM.... problem is not everyone gets new FTM's as they are released, so I'm sure this problem will continue for a long time.... even IF it is able to be fixed as they say.
I had made my tree private for some other reasons not long ago, even though sharing was my intent when I started the tree and I had loaded hundreds of photos and obits from 30 years of researching. With this new "glitch" it reinforces the private tree setting for me.
With that said, I also believe there probably are people who download, then upload to their tree and take the credit, intentional or not.