This is the way I set up the CSS. Since we are now on a fixed with, in the image is too big to fit in the width of the page, it get scaled down. However, I allow it to expand to the full size on mouse over, so you can still see the full size image if you wish.
They get bigger only if they are actually bigger. In other words, a 500px wide image won't get any bigger -- the whole image fits. But, a 1000px wide image doesn't fit, so it will get bigger when you mouse over it.
I realize it's a little unorthodox, but I wanted there to be some way to see the full size image.