That is a basic usage of the Background CSS tag
(the URL to documentation was supplied with Isaac's last post
); to take care not to completely hose your style, you should have a solid understanding of CSS before continuing with the Style Editor.
The General CSS -> Body class background:
background:#101012 url('../styles/images/chalkboard.jpg') fixed !important;
That particular instance of the tag is:
Defining a color of #101012
Embedding an image to the background: url('../styles/images/chalkboard.jpg')
Defining that the background should be fixed on the page
!important means to override any other specification
Again, this is all included on the W3C documentation.