How to I stop post images overlapping sidebar MODULARITY

the right border on my self hosted blog seems to allow images to overlap it, how can I make it so that my post images cannot overlap that border, or so that its hidden before it reaches that line.

my http://andredeveaux.wordpress.com does it but this one http://www.andredeveaux.com does not, what code do i need to add?

I just want it so that if my image is to big it will hide the remaining section that is rather than showing it going over my theme borders…