Uhm..
You don't.
The picture will tile, if it is not big enough to cover the background. Not only that, but even if you make the picture big enough to cover a 800x600 background, the first person that comes a long with 1024x768 will see a tiled image that wasn't big enough to fill the screen.
Saving the image as a .css won't help you. css is a completely different file type, and used for something completely different.