Here is the code to make this work. You will need two images in the same folder as the web page. one called square.gif and one called circle.gif
<HEAD>
<TITLE> Javascript rollover demo </TITLE>
<SCRIPT LANGUAGE = "Javascript">
function swap(){
document.images[0].src='square.gif'
}
function swap2(){
document.images[0].src='circle.gif'
}
</SCRIPT>
</HEAD>
<BODY>
<H1> Is it a square or is it a circle? </H1>
<p>
<IMG SRC = 'circle.gif' onMouseOver = "swap()" onMouseOut="swap2()">
</BODY>