I see.
There's currently no way to limit that, I'm afraid.
But you have two options available to you:
1) Modify the background image so that it has a fade to black at the bottom, and then make your backgtrounmd color black. You can do this with the CSS by this command:
background: #000000 url(http://dkpfiles.com/gitm/background) no-repeat;
That would allow for a seamless transition from the background to a black.
2) Your second option is to use
background-attachment: fixed
That would prevent it from scrolling with the rest of the page.