In your layout.css you have this:
commentform textarea { width: 45%; ....
It looks like that "width: 45%" is forcing the image width to be 45% of the page. Remove that and you should be OK, at least as far as the add-to-cart button goes.
I found this by using the Firebug plugin for Firefox and inspecting the element, then turning off styling elements one at a time until it worked.