Someplace it is written that there is an optimum size for a background image, compromising the time to download and the time for the browser to put it on the screen.
      But if you break that rule, you can "drop" a background curtain. This background GIF is one pixel high. It is a minor effect but it does give some variety.
      And the effect decreases as the speed of the user's machine increases. It will have less and less effect as faster machine speeds become more common. But, you have an effect for the slow machines and also get the minimum transmission time in all cases due to the smallest possible size of the background image being transmitted.