Jigsaw puzzles often have frame-worthy pictures on them, making them an inexpensive way to add color and art to a room. In order to frame them, though, you need some way to hold the pieces together so they don't end up in a pile at the bottom of the frame. Several options will give your puzzles stability and, if you work on the back side, won't impact the appearance of the picture.
Work your puzzle on a backer board, instead of on the tabletop, to make preserving it simpler. You can use a piece of 1/8- or 1/4-inch plywood or a sheet of foam board. These make it easier to turn the puzzle over when it's complete so you can apply adhesive. Just top it with a piece of cardboard and turn the whole puzzle sandwich over. If you plan to attach the puzzle to the board, as well, cut it 1/16 to 1/8 inch smaller than the puzzle on all sides, using a sharp utility knife and a straight-edge, such as a metal ruler or carpenter's square. That way, it doesn't show in the frame.
Glue It Up
After you flip the piece, paint the back with decoupage medium or with equal parts of craft glue and water. Allow it to dry and then add a second coat. If your frame doesn't include glass for the front, you can protect the colors of the puzzle from fading by adding a couple of coats to the front, as well. This changes the appearance slightly, so you may want to use a high-gloss sealer or variety of the decoupage medium instead of a matte version. Allow the coating to dry thoroughly before you proceed with framing.
Stick It Down
Clear contact paper applied to the back of the puzzle will also keep it intact. A special peel-and-stick puzzle sheet is also available, but any type of contact paper will work. In fact, because it will not be seen when the puzzle is hanging in its frame, you can use printed contact, as well. It can be difficult to work with large pieces of the sticky stuff, so cut smaller pieces and overlap them slightly when you add them to the back of the puzzle. Work slowly and smooth out bubbles and wrinkles as much as possible.
Back It Up
Substitute the board you used to flip the puzzle for the flimsy backer in the frame by gluing the puzzle you've preserved with one of these methods to the board with wood glue. Allow it to dry thoroughly before slipping it into the frame and securing with the pins or tabs provided with the frame.