[How to] add Javascript to CSS

[gamerfu]

I want my images to rotate, but cannot quite figure out the CSS function.

I have this:

.headerL {
	height: 180px;
	float: left;
	width: 309px;
	background-image: url(images/styles/1ATKSUM/misc/logo/left/0004.png);
	background-repeat: no-repeat;
}

but want this:

.headerL {
	height: 180px;
	float: left;
	width: 309px;
	background-image: url(<SCRIPT LANGUAGE="JavaScript"> 
    <!-- Begin
    showLeftImage();
    // End --></script>);
	background-repeat: no-repeat;
}

Ideas?