This is what I would do :
PHP Code:
ShowImg = function(img){
var background = document.getElementById('background').style;
var backstep = 'url(images/bannerthin.png)';
if(img) background.backgroundImage='url(images/' + img + '.png)';
else background.backgroundImage=backstep;
};
HTML Code:
<div id="background"></div>
<a href="#" id="1" onmouseover="ShowImg('banner')" onmouseout="ShowImg(null)">1</a>
<a href="#" id="2" onmouseover="ShowImg('text')" onmouseout="ShowImg(null)">2</a>
<a href="#" id="3" onmouseover="ShowImg('bannerthin')" onmouseout="ShowImg(null)">3</a>
One function that does it all, swaps the image if the parameter isn't null, resets it to default if it is.