Note: I have no idea what one person meant by saying their images were hacked and used on a porn site? Just what kind of photos does this person have on eBay? I have NO photos of myself, and eBay requires no photos of yourself. So I have no idea what this person is talking about. You do need a photo of the item you are selling. But if handbags and porcelain plates are the type of thing that arouses people on a porn site, then maybe we are in the wrong business!?
This happened on Etsy to many hundred sellers. What kind of images? Handmade good such as jewelry, supplies etc. On a porn site? Yes - hackers used these images as bait to get more hits for their site. So no, these were not images of people (for the most part and not "those" kinds of images). Idiotic, yes but if you sell online you have to be ready for some unexpected things.
Overall, I have to say that beyondthestargate's advice is right on and I especially like the blacklight marking system.