How do I mount a canvas picture in a frame that is larger than canvas?

Purchase matting that fits the frame and compliments the colors in your painting. Then mount the canvas to the mat board. A proper framing store will provide tips. I would just carefully glue it to the mat.

Put fabric or wall paper or whatever you want on thin wood to use as a backing in the frame. Mount the canvas to this wood so that is it centered. My mom used to do this when she mounted things like coins, belt buckles and other decorative objects in frames. The thin wood will provide the structure to hold the stretched canvas that the usual cardboard that is in frames won't provide and it also will take whatever you put on it with out falling apart from glue or adhesive spray. I hope this helps you be able to hang your art work.

You can make canvasses in custom sizes by purchasing wooden stretchers in the sizes you need, then stretching canvas fabric over the stretcher frame. Craft stores and art supply stores sell both the stretcher frames and artist canvas.

My husband is an artist and in a situation like this, he would stretch his own canvas to fit the frame. I think all the stretchers' and canvas would be available from art stores. Or if you like you could just hang the canvases on the wall and hang the frames over them. Not ideal but one way to display both!

Perhaps you can fill the old frames with thin plywood or underlayment. Then cover with fabric or art paper or just paint the wood the same color as your wall and mount the art to the plywood. If you use underlayment it will need to be sealed before painting because it has a lot of tanin that doesn't agree with most finishes. Also, if you paint, you should paint both sides of the wood or it will try to curl/curve around the painted side.

Have a mat board cut, and use a gentle adhesive on the sides of the pic to slide mat against and hold it.