randomly changing images when hit reload/refresh

hello
just wondering if there is any way to make the header background image to randomly changing everytime i hit reload/refresh? im using butterfly theme from http://www.sheadetoptions.org

#header {
margin: 0;
width: 800px;
background: url(images/head.jpg) no-repeat;
height: 150px;
color:#a3b2ba;
}