which version of cubecart are you using?
I think it automatically stacks the images into columns as you add more products.
I think it's based on the width of the latest products container and the size of your thumbnails.
So if the width of your latest products area is 400 pixels wide and your thumbnails are 175 pixels wide, it will give you two columns of images.
So if you add one product, it will show it in the first lefthand spot, if you add another product, it will show it to the right of that first image. If you add another product, it will show it right under the first product in the first column.
Here's how they look side by side in the cc demo:
Blue Sun Corporation - (Powered by CubeCart)