The Header of your site can easily be changed by going to in your Dashboard → Appearance → Header
Ideally for your site the header image should be 936px in width if you keep the Wrapper padding at 30px anda border of 2px. If you change the Wrapper padding to 10px and the border to 4px your header image should be 972px in width. The header may be any height.
You can of course add any width header image into your site and the theme will automatically adjust the header width and the height ratio to fit.