The problem is with this image:
It looks like you have replaced this image with your own custom image.
In order to do this correctly, you will need to slice the image correctly. If you look at this image:
you could see that at the right side, you will need to have the shadow included as part of the image.
another way you could do this is by putting back the default image, and then adding the img inside the <div id="header><img src="yourimage" /></div>