Alright, I gotta know, whats the poster?

Usually there are universal keys like the triangle ones. Depends on your country. Usually these frames can be opened from the sides so look there for any key holes. Get a thong and open the triangular thing sticking out. It's the lock mechanism. Then try giving each side a strong pull to see where it opens from

if you really don’t wanna steal it then you could just try getting a really good photocopy of it onto poster paper