If you have one image it Photoshop it will not resize to fit the window. Images processed through Photoshop have a specific number of pixels wide x height. If you expand it you will stretch it.
Depending on your image a few things could happen.
1. You want your header image to the left. If this is the case you will need 2 slices in Photoshop, main image on left slice, repeating image on right slice. Then when you place your div, align your image to the left side and use a repeating background with your second slice ( http://www.w3schools.com/css/pr_background-repeat.asp )
2. If you want your title centered, use the same as above, although your image needs to be centered by placing it inside a div within your header and setting the margin: 0 auto; . Then the repeating background could be from either end of your Photoshop image since both sides would then be the same.
3. If you want your header to the right, see #1, but reverse alignment to right and change slicing in Photoshop accordingly.
By default, images are static and don't resize to fill viewport. If however, you put your banner into the background using CSS, you can repeat the image horizontally, vertically or both.
See more on backgrounds:
Another trick is to surround your banner with compatible colors to make it seem bigger.
Liquid Header CSS Layout: