By going just a couple more lines in the code, and changing the numbers following the example you gave, I was able to get it to work.
However, it's doing the same thing that happens when I change from 3 lines to 1. The "more" link on the right side disappears after one click and you can only go left. Even after you click on the left "more" link a few times, there's still no "more" link to the right.
Since it's a hack, you're obviously not going to debug it.
So, I'll close with this: Please consider adding the option to vary both the number of rows and the number of columns of thumbnails in "showphoto" in a future version. Thanks!