A picture is worth a thousand words, but those words won't say what you'd like if your picture frame is warped. Picture frames become warped when stored improperly for an extended period of time, or if the frame is made of wood. A wood frame will take in moisture and cause it to lose its shape. While this process will take some time and patience, you should be able to regain the original shape of your frame by following a few steps.
Remove the backing of the frame, the picture and the glass.
Place the frame on a flat surface, face up.
Apply pressure using clamps or by placing a heavy object on the warped areas. This process will take patience, as you'll want to leave the clamps or heavy objects in place for a couple of days.
Add Velcro along the back of the frame, and align it to the wall to reinforce the straightened shape.