Part 1 of x - CssInstructions
Find this section in your css
/* xxxxxxxxxxxxxxx */
/* images */
/* xxxxxxxxxxxxxxx */
html, body {
background-image: url("http://static.flickr.com/36/123759757_52c8587728_o.png");
background-position: center top;
background-repeat: repeat-y;
}.
div.header {
background-image: url("insert_url_for_your_banner_here");
background-position: left top;
background-repeat: no-repeat;
replace insert_url_for_your_banner_here with the url of your new wiki-banner (for example on http://www.flickr.com or elsewhere). Leave the “ at the beginning and in the end. Save your css.
If everything went fine you should see your wiki-banner now