Find your styles.css file for your skin (it might be called stylesheet.css), depending on your skin.

For some of the skins it's located here: /skins/yourskinname/rsc/styles.css

others it is located here:/skins/yourskinname/stylesheet.css

Look for this:

body {

It might look like this (if you don't have a background image now):

body {
background-color: #FFFFFF;
padding: 1ex 2ex;
margin: 0;
color: #000;
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 84%;
}

Change it to look like this (you are only changing the background line):

body {
	background:  #EFDBB5 url(img/bgtile.gif);

The color spec shown there will default if the image doesn't show.

You can also write this another way:

body
{
background-image: url('bgdesert.gif')

By not having anything to designate the location in the code, the default is to tile, completely filling your background. However, it is also a correct css syntax to show:

body
{ 
background-image: 
url('bgdesert.jpg');
background-repeat: repeat

But, that "background-repeat:" tag isn't really necessary if you want it to "tile." Try it both ways and see if there is a difference. The background repeat tag is useful, however, like this:

background-repeat: repeat-y (tiles only one unit vertically flush left)
background-repeat: repeat-x (tiles only one unit horizontally at the top)

There is a lot to learn about what you can do with css. Here is a useful site for learning the various tags and syntax:
http://www.w3schools.com/css/tryit.asp?filename=trycss_background-position

Sam