My pictures are coming out unoraganized

I have been posting completed projects to our website. Sometimes, I will add 6 photos. They should come out with 3 on top and 3 underneath. Somehow a a random picture from the selected media will find itself on a 3rd row. It looks very unorganized and I would like to know, Why is this happening and what am I doing wrong?

The page I need help with: [log in to see the link]