Hi Andrew:
To add an image to the background, you need to add it to the body tag.
In your CSS, add something like this:
body {
background: #f2f1ec;
background: url(url-to-your-image) repeat;
}
You can control the positioning, size, repeat and other things related to the background image in CSS3.
Please refer to
www.w3schools.com/css3/css3_backgrounds.asp
and
css-tricks.com/perfect-full-page-background-image/
for more information.
Also, make sure you are using a custom CSS file or a custom less file. There are lots of forum posts on how to properly configure the custom CSS and less files.