put a new picture(poster,paint,contact paper, etc.) on new canvas with spray adhesive(if you need to attach something like poster) and stretch it over one you want to cover then use staple gun to attach to back of frame
Janice, cover them with a pretty fabric you like, just staple onto the back. Here are some examples.
Thanks Rose, video was a big help, now i know how to do corners. Going to look for fabric!