The best way to solve this problem is to use CSS.
Inside the CSS class you can specify the background property which will take the image for a background or, if the image does not exist, it will use the background color instead. Example:
<style>
.myClass
{
background:url(Images/top_banner/bck_top_banner.gif) repeat-x top #DDDDDD;
}
</style>
Then you have to assign this class to your link button.
Try this js code, though I am not sure about it:
document.getElementById("LinkButton1").attributes["class"]="myClass";
I would rather use JQuery to handle all javascript.
then it would look like:
$(".myClass").attr("class","myClass");
<pre />
good luck.