Unfortunately Teeworlds development can't help you on this one. Although the police might be your best shot at this one, there are some questions which you issues which you probably need to solve before hand.
1. How did they get to your picture, was in on the internet, or did they access your personal files? If it's either one of these, this would add an extra complaint of hacking or copyright.
2. Why would they use your picture on their background? Do you know them personally from school, work? Then it might be good thing to speak up to your teachers, or other supervisors.
3. Why would they try to grief you in the first place? Did you misbehave on their servers? Did you insult any of their clan? If so then a friendly excuse might be all that's needed for them to clear this problem.
These are just some of the questions which might provide the best way out of these troubles. There's a good chance they don't apply to your situation but there has to be a motivation for their behavior. When that's known there might be other possibilities.