[@goebelmasse Member 16 years, 3 months ago]

By editing the style.css file.

body {

[...]

background-image: url(path/to/the/image/relative/to/the/css);

background-repeat: no-repeat; /* or whatever you like */

[...]

}

Of course I cannot give a full CSS introduction here, but there are lots of good and helpful introduction available online.