Background images are automatically "tiled" to fill the users browser. If you search some of the free graphic sites for background images you'll see that they are very small - usually around 120x120 pixels - depending on the image.
If your background image is, for example, a house, and the image size is 100x100 pixels, you're going to get a lot of "houses" for a background. But if you want a single house, you'll have to create the background image big enough to cover the users screen and monitor resolution.
Let's say that you want to make a background to fit a screen resolution of 800x600 - you're image will have to be approx 790 pixels wide and at least 500 pixels (depends on browser tool bar settings) tall. Of course, if you're page scrolls down, you'll have to make it taller or use CSS to make the image "non-scrollable".
There's always a better way...