Background image won't stretch

I am uploading an image (4493 x 3000) through the “Theme Options” tool for my background but, it looks horrible on all screen sizes. If it is on a blank page, it doesn’t appear that bad, you can kind of tell something is wrong on the footer. On a longer page, one that requires scrolling, the image won’t scroll or stretch. Not sure if I just need to re-size my image, or add new css code?

I would prefer the image to scroll as the user scrolls, so it appears one image and you can always see the whole image.

http://www.alitusgroup.com