You can post anything on Pinterest whether it's your photo or not. It's purpose is for "re-posting" other's photos. Some sites encourage you to post their photos on Pinterest as it sometimes increases traffic to their site. (Search engines are now programmed to include social media hits into their algorithms for ranking sites.) Food sites are particularly happy to get their photos on Pinterest.
There's not much you can do about it. A friend had a photo "stolen" from a post on Gardenweb and published in a brochure. She contacted the nursery who said they paid someone to find them the photos and they would not delete her photo. I had a photo posted on Gardenweb, that I also posted to the
hostalibrary.org (which specifically says that photos cannot be used other places without the permission of the donor/owner) that was used in an online catalog by a hosta vendor. I knew and liked the vendor, so I did not complain. He's since taken it down.
Again, there's not much you can do about it. QB is really picky about posting photos, but most sites are not. Pinterest and Instagram encourage it. The "bots" sent out by the search engines vacuum up all photos posted anywhere. The search engines then post them online, making them easy to find so others can help themselves.
bkay