Yes, the keyword you are looking for is "watermark"

Did you read the story of 2011's storming of Bin Laden compound? And that they found heaps of porn videos? And that's how he exchanged messages with his network by using different open porn sites where they uploaded porn? I recommend reading it, it's quite interesting.

Where is the link? Well, I don't give 100%, I like to only point people in the right direction, rest is left as an exercise for the reader.

Here is the link for less obtuse people: https://www.nbcnews.com/id/wbna47254281