Image Height set to crop, how do I change?

I bought a dynamic theme that uses a “feature image” box at the top of all posts. This box has a set width of 612 px and height of 400 px and it automatically crops any image that is taller than 400 px. I want to keep the width at 612 px but have the flexibility to upload an image of any height and not have it crop. Is there some CSS that I can paste into my Stylesheet Editor that will fix this?

Here is a link to the problem: http://weeheart.com/?p=128