Why do my thumbnails in my image gallery become vertical when I add a new image?

I have my image gallery positioned so that the images are positioned in twos and stacked one on top of the other. However when I try and add an additional photo the grid changes to one on top of the other. Why is this happening. Even when I try to re-position it with the blue lines it reverts back one image on top of another rather than two image side by side.