The problem is that even if you were to center your image header you presently have as your header image - there would still be an ample amount of space or "gaps" ( padding ) left over on both sides.
The main issue is that your header image is only 960px in width - and your widget header is 1210px in width ! Thus, your image will display as halfway cut-off.
My advice to you is to find a much nicer image - scale it to 1210px wide, apply your header text on it ( or your logo along with your tagline ) and implement it. However- the image WIDTH must be 1210px.