I found the 16th square. Just to clarify, the diagram is drawn on a grid 7x7 (you can visualize and count this if you start in the top left and count each of the smallest squares diagonally to the lower right). The little yellow squares are 1x1. I kept missing the 5x5 that includes the top left corner because I perceived it as a rectangle, but counting the smallest squares reveals that it is in fact a 5x5 square.

Hope this helps other see all the squares.