[Plugin: NextGEN Gallery] Thumbnails are all different sizes? Please Advise

Great plugin! I just have one problem, the thumbnails on my gallery page are all different sizes? Is this something that I did wrong or can change in settings. Please take a look at the page @ http://www.nerdyguitar.com/nerdy-guitar-gallery/ (you will have to scroll down the page a bit the see them change sizes). I like the middle sized ones and want them all to be this size. I would greatly appreciate some input here.

http://wordpress.org/extend/plugins/nextgen-gallery/