The trouble with getting a background image to cover the "whole page" is that the whole page will be different sizes for different users, depending on their screen resolution. For example, an image 800 x 600 pixels will look fine on a monitor set to 800 x 600 or smaller, but view it on a monitor at (for example) 1024 x 768 and the image will tile to fill the available area. You could of course create an image of a ridiculously huge size (say 5000 x 5000) which would not tile on any current monitor resolution, but then you would have the additional problems of large filesize, meaning a slow page load time.
I think your best bet if you really
have to use a background image (and many professional web designers wouldn't touch background images with a 10 foot pole) would be to create a proper seamless tile. There are a number of tutorials available on creating seamless tiles. Here's a start:
photoshop seamless tile tutorials - Google Search