I have a Bessey strap clamp and don't have much trouble with it. In fact, I just used it on a box last week. Note, I said "box"?! I don't think it would work too well on a picture frame unless the frame was at least 1-1/2" thick. For picture frames I usually use the metal corner clamps or a special frame jig that I've built. Some times if you can clamp one side down to the edge of a corner of your bench and another side along the other edge you can use a square to true it up while the glue dries. (put wax paper on the bench so you don't glue the frame to it) If you build the frame two corners at a time (opposite corners) then do the final two corners at the same time to form the rectangle to finish it up. Another thing to take into consideration is the size of the frame (8" x 10" or 24" x 30")
They do make special picture frame clamps that are used in professional frame shops but, they are fairly expensive for most of us hobbyist. That doesn't mean that you couldn't look at a few pictures of the pro clamps and copy the idea and build a wooden one!

Rog