Thanks for contacting us! You can add rows and columns by using the Rows and Columns sliders down the bottom left when creating a collage.
You can also change the shape of the collage using the Proportions slider.
Starting in the next fortnight or so we will be launching new functionality which allows you to insert cells, rows and columns with even more flexibility, as well as resizing the entire collage, so stay tuned for these updates.
Let us know if you have any other questions.