To date I am not aware of any of my family photos being copied to Pinterest, but it wouldn't surprise me.
There many people who as Ros says, download, copy or screen shot a photo in order use it on another website or on a person's profile on a family history site.
I always put a source and the names of the people on the picture.
Like this one https://www.wikitree.com/photo/jpg/Richards-13892-2
On Ancestry a person can save a photo to their tree, but as has happened several times to me, a person has downloaded a photo I have posted, but then has uploaded the same photo saying it was their picture.
The offenders have edited the picture and removed the source etc I have on the picture.
The only way I have found of partly protecting pictures is to is to put a watermark on the picture in the centre of the picture you uploaded, meaning that anyone you wants to download then upload a picture would need to edit the photo to remove the watermark, but the result for the copier would be removing most of the picture.
Google search ' how to watermark a photo'.
The flip side of this issue is that many people consider that photos are part of family history and therefore should be shared.
They are part of a person's family history but copying etc without source information and attribution is in my 'not so humble' opinion bad manners.