Are your photo squares larger or smaller than your U of F squares?
What you need to do (to keep life simple) is to determine what size your photo squares need to be so you don't cut off the image.
If they are smaller, then you can add a fabric that blends nicely with your U of F squares to frame them out to the 5 1/4 (you want to do your best to make sure all your blocks are as close to square... and the same size as possible... or you will go crazy sewing them together!).
Let's say your picture square is 3 1/2 inches, for example. To get to 5 1/4 inches you will need to add 1 3/4 inches of fabric. And if you want to frame the photo square -- you'll want to divide that fabric so you will add some to both sides of the block. You also need to allow 1/2 inch on each side for seam allowance (assuming you are sewing with the quilter's commonly used 1/4" seam allowance.).
So you take your 1.75 and divide by 2 and get 7/8 of an inch. Now you add the 1/2 inch seam allowance which gives you 1 3/8" If it were me... I'd cut the strips 1 1/2 inches... sew them on (using 1/4 seam allowance then square them up by trimming on the edges if necessary.
Does this make sense? It really isn't hard to do -- it is just hard to explain in text!