Fitting your front page image.

Whenever I try to use an image as the side background on my front page in the boxed layout it doesn’t scale to fit properly. As an example as posts start going up and then I scroll down the image doesn’t stay static so when I scroll I will reach the edge of the image where it starts over again as a tiled type background.

Is there an image size that works best? or is it a setting that I am missing that will lock my front page background image in place as well as scale it to my page size?