You can’t get much easier than this. Go to Advanced Options and click on WordPress 3.0 Features.

You should then see this screen. Just check the box and hit Save.

You should now have a new option in your Appearance tab called Background.

Click on Background and you should see the screen below…

Once you upload a new background image you can set it’s position, set the repeat, make it scroll or fixed and also set a new background color.

That’s it. The only limitation would be if a background gradient is present. That tends to overlay the actual background image. A background glare or light effect seems to work ok along with the new background image.