Automatically watermarking posts with images

I've searched the sub, and googled, but could not find a solution to the following.

My sub has had a lot of content posted on FB pages and others , and I was wondering if there is a way to automatically add a water to post's images with my sub's name across it. I know this is all public, if people are going to take our content I would at least like to have a water mark that leads them back to us and gives us more users.