Page layout issue

I’m trying to make an archive page for a web comic:

http://timarsenault.com/birdandmoon/archive/

I’ve uploaded thumbnail sized images with captions. I want them to have an even number on each row but for some reason they will break at weird points and have some rows with only 1 or 2 images that are left aligned. I can’t for the life of me figure out what’s making these weird breaks.