You need to manually extend the header to the width of your page. To do this you will need to have fireworks or photoshop software or some other graphic software. Trial versions of Adobe software will be fine for this task.
Once the picture is wide enough, upload it to your website by going to:
Appearance >> Header
To widen the image, you simply create a canvas of the correct background color and correct width/height and then import your image on top of it so that it rests on the left side while the right side will simply match with the rest of the image.