A static image will never scale properly. there are several methods to create a "fluid" header but retaining the exact look as you have now would be difficult.
If you think that the majority of your customers will be at a resoultion of 800 or higher, then your could separate some of the parts of your header. Like take the title out and make it a logo or sales message. make a left image and a right image with the center part just a color and use the css sliding window trick(see the future zen template 2.0 for an example of this). Or you could redesign the head image to allow a trailling repeating image that would fill out another resolution.
Bottomline, create for the lowest res that is prvalent and make fluid for the others.



