Get Imagezoom for Firefore. With a right click you can adjust image size (or right-click and use the mouse scroll wheel), zoom, and rotate images (useful for people that can't mange to rotate their pics in paint).
The forum auto size feature is nice, but the Firefox plugin can be used on any website.
And I only recommend that since I have no idea how hard is it for someone here to do the auto-fit thing, nor how lazy someone would be about it.