I think you may need to clarify the question a little.?
You can always use a wider background image in an element because the background is always the width of the actual element and the image will just be clipped at the elements dimensions.
If you want a bigger element then as Ralph said you will need to make the element bigger or use another element.:)
Or if you want to scale a foreground image then you can do as Tommy suggests but that really depends on the image as some images do not scale nicely.
If you want a header that repeats all along the top of the viewport you could apply it to the body element and repeat it on the x-axis.
It all depends on your exact requirements:)