What if someone stole your avatar/username?

What if someone stole your avatar/username?

On one of the forums where I was posting five years ago, another user took my real person photo and real name and used it for themselves. They simply copied my name and photo and started an account on a related forum so that it looked as if I was the one making the new posts. This was a very troubling situation considering how they were also trolling in my name. Many of my friends on the forum thought I was the troublemaker though I had not registered on the second forum.