body {background-image: url(picture.png);}
This will cause your browser to look for a file (in the same folder as your HTML document) named picture.png. This is a form of relative linking. Another example would be to add a sub folder before the actual picture: url(/images/picture.png) You can also use absolute linking to set an image:
body {background-image: url(http://www.newbtopro.com/images/picture.png);}
Regardless of what way you link to your picture, your image will show at the top left of the screen.