Foren / Cotonti / Skins / random logo css

Hello I want to load a random logo in my header but I have no idea how to

This is what I have in my css............

#header {
background: url(../img/logo2.png) no-repeat ;

background-color: #113355;
     width:1000px;
     height:181px;
    height: 278px;
}

and this is from my header.tpl............

<div id="header">
 

</div>

How can I get it to load a random image ?