Adding image outside theme to hide extended background

Hello!

I’ve probably worded my problem terribly, but here goes:

I’ve created a super long (2500 px) background image so that it doesn’t “run out” when someone scrolls.

I want to insert a 40 px white box at the bottom of every page, below the footer, in the margin of the theme, so that I can hide the background image and give the illusion of the border ending at the footer.

Here is the site: http://radiusensemble.org/

You see how the vertical lines continue below the footer? That’s what I want to hide.

I’m using a highly tweaked Weaver 2011 theme.

Yes, I did try removing the border from my background image and using the site border function but it doesn’t achieve the effect my designer was going for.

Thanks for any and all suggestions!