I have leftover glass from frames. How can I use them?

Be careful with how much pressure you use on this glass. It was not meant for pressure. Also be careful because if it does crack, it could cut you easily if it is in shards. Picture glass is not safety glass.

That said, I have used bulletin boards of cork to mount posters and pictures with Plexiglass. You could find a framed cork board and put pictures in them this way.

You will need glaziers points. You can get them at a hardware store.

Place your picture(s) on the cork and then lay the fitting glass piece into the frame. Insert glaziers points at the sides and corners to secure the glass inside the frame. Hold it up vertically over a soft surface to see if it is holding properly (and if it isn't, the soft surface will help prevent shattering). When you are sure it is secure, hang the bulletin board frame as a picture.