I think that Grannie Annie's idea is the easiest for a beginner. I doubt that there is a pattern that will specifically deal with the sizes of pictures you have. One thing I did with photos was to put several of them on one page of the fabric that I printed...because they were different sizes, sometimes I got three on a page, sometimes I got four or five. For one large one, I made it the only one on the sheet of fabric. The fabric I use is Printed Treasures (I think that's the name) and it is 8.5 by 11 inches so it fits into the computer. Leave enough space around the edge to have a seam allowance and use that rectangle as the block size...if you prefer to use a square - add strips of fabri around the edges to make it 12 inches square..and yes, the fabric strips are called sashing.
PM me if you have specific questions.