Thanks for reaching out to us!
If you select the 2nd Jigsaw layout (on the top row), you can then insert an additional cell by dragging one of your photos from the Photo Basket (at the bottom of the editor) to between the top 2 cells. A blue rectangle will appear, indicating that it’s going to insert a new cell – then let go of your mouse button to insert it.
Let us know if you have any further questions!
Regards – The Ribbet Team