Best size for background image

HI,

I have a holiday site http://www.silvercoastlife.com that is focussed on beach life and the sea and so would like to have a background image of sparkling sea scrolling down to a beach background behind the footer. This would replace the current rather dull faded sand image. Is this hopelessly ambitious. If so I will use an image of ‘sparkling sea’ and tile it to fill the space. Can anyone advise me of the best size to keep page load times at a minimum while not having too many ugly tile joins showing up? I guess the depth of the pic is most crucial here as the joins will show on larger screens. Any advice would be much appreciated.