I used Google Developer Tools to look into this & the position:fixed doesn't seem to address the issue. When that is applied, it no longer stretches the image to fill the width of the page. It releases the image to revert back to its original size. The width on .sf_header_wrapper needs to be set to 950px. This makes the image stretch to fill the entire width as before.
However, by making the header image scroll down the page, you are covering up the menu on the left side of the page. Is this what you want to happen? There also seems to be some interference with your navigation being functional when you make these changes. Unfortunately, this is one of the drawbacks of using templates, they can be very frustrating to get what you want accomplished, they do come with a certain amount of limitations.
Hope we can get this solved for you!