Digital signature. A watermark may be useful so that an unauthorized user can’t easily hide their source without noticeably defacing the photo, but it doesn’t prevent anyone from modifying it
A digital signature is a somewhat similar idea except that signature verification fails if there are any changes. This is tough to do with a photograph, where some applications may be blindly re-encoding or modifying the resolution so those may need to be fixed.
You could argue this is a good use case for blockchain, certainly much better than those stupid monkey images. When John Stewart parodies a politician, there should be a verifiable chain of evidence from the White House release to the news bureau to his studio, before they alter the lighting to highlight orange skin tone for yucks.