There a girl in my school who put up a display. A few of the pictures were rip offs of what i have seen going around the internet.
When you say 'rip-offs' do you mean she has actually used pictures that are not her own? Or has she just stolen the idea?
You say you have seen the pictures going around the Internet.
Did any of these images have details of copyright?
If not, then the people who posted them on the Internet have probably breached copyright.
The girl may have picked up one of these and not been aware of the copyright breach.
You have to be very careful in situations like this because you yourself could end up in trouble. You have to not just prove a breach of copyright, you also have to prove intent.
If you make claims you can't prove you can fall foul of libel/slander laws.
It seems to me you only really have one course of action.
Tell a teacher.
It's their job to deal with stuff like this so let them.
They may not be aware of the situation so if you tell them of your suspicions then they can, at the very least, question the girl about it. Which may well discourage her from doing it again - and it will make the teachers more vigilant.
It will also benefit you.
Your conscience will be clear as you will have taken positive action. And it will save you a lot of time and frustration as well as avoiding putting you in a difficult position.