Resize background proportionally

I love this little plugin — it works great. the only thing I dont like is that it LITERALLY fits the background the the window, with no regard for keeping the correct dimensions of the image. If you resize the window, it will distort the picture. Is there any way to resize the background proportionally (keeping the correct/original dimensions)?

Thanks!

http://wordpress.org/extend/plugins/simple-full-screen-background-image/