Picture framing

I found a beautiful Christmas puzzle scene at a flea market. It certainly needed a new frame and some cleaning up. Now the picture was covered with "plexy" sp) glass. that is still fine. The problem is the frame had to be changed. We used a piece of cardboard to the back and have a piece of molding to make the frame, my problem: How do we hold the picture and glass in place? The frame will be on the outside of the picture attached to the cardboard.
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