TwentyEleven Theme Header Image Size

The header image in TwentyEleven Theme is a default size. It seems really big to me and I would like to put one that is a smaller height. If I put a smaller pic in, it stretches it out, slightly distorting the image. I want the header image the same length but would like it to have a smaller height. Is this possible? Thank you. The site is http://www.polyliving.net