Script Summary: Bypass Google's image lightbox and open images directly at their actual resolution.
This is a script I've been testing for a while now, and I think it'll do you well.
It bypasses the lightbox that Google usually uses when you click an image and shows you the actual image at it's URL, so it'll be in full resolution.
There are no settings to mess with, and nothing complicated to change. Just install it and it'll work every time.