I gather you mean you want a picture in the background of the Main Panel. To stop it tiling, go to Other > Global Scripts > HEAD section and add this CSS:
Code:
<style type="text/css">TD.XSP_MAIN_PANEL { background-repeat: no-repeat; }</style>
To attach the background image to a certain part of the Main Panel, go to Other > Web Site Settings > Image Alignment > Main Panel. Set the alignment as you wish. Background images should be sized to fit the background width in an image editor like Photoshop. The height will be unpredictable, so fade the image to white at the bottom (or the top, depending on where you align the image to) for the best effect. Or perhaps the image doesn't have to fill the background, as in the following example:
http://www.hiring30.com/. This site has a pale image in the bottom right corner of the Main Panel, but it doesn't fill the Main Panel. Note: It's the same image that's in the header.