Header Problem

Hi,

I’m using the twenty thirteen template and I’m trying to change the header to a 1200x173px sized one and wordpress is stretching it to 1600x230px while keeping it inside a 1200x230px shape. That means that the sides of my header are being cut off from sight and it’s stretching the height.

Can anyone help me. I’ve tried to change it in the header section of the style.css page and custom-header.php but nothing is working.

Any help would be appreciated