Inconsistent thumbnail sizes using wp gallery

The thumbnail sizes using the wordpress gallery are inconsistent on my page. http://anitamorris.net/artwork/

I’ve already turned the plugins on and off. I’m currently running only three. Akismet, Jetpack and BoostSuit none of which seem to cause the problem. I’m running twentytwelve.

I’m building this site to show my art which means everything needs to consistent in order to appear professional.