The CSS tab contains the CSS code that defines the look and style of your website.

Referencing Images in CSS
To reference an image in CSS, upload the image using the “Add New File” link and use the image name as the URL path.

Here's an example CSS property that references the image file "pattern.jpg": 

background: transparent url(pattern.jpg) repeat-x center top;

Including a Theme Header Image

To include a header image in your theme, include a div tag in your HTML with the class name "wsite-header".

Ex. <div id="header" class="wsite-header"> </div>

If you include this <div> tag in your HTML, you can specify a theme header image in the CSS with a background property.  The image location should be referenced with the variable %%HEADERIMG%%.
For example, a header image CSS rule will look similar to this:

.wsite-header {
  background: transparent url(%%HEADERIMG%%) no-repeat center top;

The default header image should be uploaded to the “Files” tab with the name default_header.jpg.  You'll have the option to change the default header image using the website editor.