Lila, my 12 1/2 in ruler's inside square is only 6 1/2 in. My next largerst ruler is an 8 1/2 and when I center it on the picture, the picture is clearly not square. It is wider than it is long. Sorry, I am not following as far as with the other suggestions. Tape off both ways from the corner of a larger square, putting the corner at the far side corner of the picture?