Tape little paper sheet inside the glass frame using clear scotch tape. Upper part of this little paper sheet should be in the place you want your posters bottom to be:
Put the poster on its place, and flip the little sheet to the back side of the poster:
So, from the back it should look something like this:
It should look nice from the front after you put the background on. Be careful when you turn the frame, so that the poster does not slip from it place.
This tip is not suited when dealing w/ posters w/ thin paper. As one can see the little paper through the poster:








