Ahh...Try this Felix. Take your mouse to the bottom corner of any website and then click and drag going upwards diagnally to the top left corner. You'll notice that some sections are darkened while others remain the same.
With UBB you'll see the left and right of the banner there will be 2 darkened images. Those are 2 separate images (one aligned left while other aligned right). The middle part is just a background color to tranisition the images as if it were in one piece. People do this to compensate for different browser resolutions and screen size.
So in a nutshell UBB's banner is comprised of a left image and right image.
