Site Background Image

Hi,

I have added an image for my site background, I want this to be centred and positioned at the bottom of each page - so that I can set the site background colour as the blue colour and it will blend in to the sky on pages where the page is higher than the image.

On 'theme options' it looks like I have set this up to happen, but the image just sets automatically to the top left of the page.

My site is at 4.biz. A good page example of why I want my image to be aligned centre bottom is on 4.biz/blogger-package.

Is there some code I could enter in the .css that would get around this problem?

Thanks.