help! Background Size, how to make background stretch Options

[infernos-curse]

Hello, im having problems designing a layout for a friend

i made a background, my resolution is 800 x 600 so i see the image cover the whole page

but my friend has it 1024x768

so when i changed i saw that the image doesnt stretch completely to fit the background on both resolutions,

is there any code to make my background stretch depending on the resolution of the user?

or anyway to fix this?

heres the page:

http://www.myspace.com/frez_co