I use Tineye.com

You can save a photo from a provider and incorporate it into tineye which will then search the internet(alot of it but not all) for an identical photo. This has helped me many times discover that a providers photo is sometimes a photo taken from another person or website.
Its not a full proof system but a good step into researching an ad. Also you should note that finding a picture of someone on multiple sites is not a bad thing, but depending on the content of the website it could be suspect.

I suppose that providers can use it so see if anyone has stolen their photos as well.

I hope LE doesn't find any nefarious uses for it however.

It's the same technology used to find folks peddling in, shall I call them, illicit images.

They get one persons computer and their collection, and they use similar technology to find other images on the web. Very effectively, and I am glad for it.

I know I would be very upset if I found my pics on another providers ad or website.