As far as I know, there is no way to do this using CSS, which is what I believe you are using to display this background. The only option that is given is to repeat the background until it fills the screen (in x, y, or both).
Stretching may not be the best option, considering there are some very wide screen high resolution monitors out there that could double the width of your image and distort it a lot. The maximum I found for a monitor width is 2560px. If your image can't stretch a normal 1024px you have to stretch it 1.5 times it's normal width.