Background in a site Help please

Like if you have a background that's 500x500 and you want it to adjust itself to fit the screen no matter how big or small the image is its gonna take up the whole page
And i can only put it in the body section but i want it to be background i cant edit the <body>
Can you guys please help
Imran Hashmi
www.visionstudio.co.uk
www.seo-professional.co.uk 0044-7969012441