Welcome aboard. I wonder if the puzzle isn't square (90-degree angles at each corner). It's not uncommon for jugsaw puzzles to be out-of-square. There may be a little distortion from shooting wide (24mm). As far as preventing it, if the problem is that the puzzle is not squared up, you could try a pair of inexpensive drafting triangles (45-90-45 degrees and 30-60-90 degrees). Place them at opposite corners lining up with 90-degrees and use a straightedge (meter-stick) to try to align the sides of the puzzle.