Try this


<script type="text/javascript">

function doIt(obj){
document.getElementById("bigpic").src=obj.src
}

</script>


<img src="pic1.jpg" width="100px" height="100px" onclick="doIt(this)">
<img src="pic2.jpg" width="100px" height="100px" onclick="doIt(this)">
<img src="pic3.jpg" width="100px" height="100px" onclick="doIt(this)">
<img src="pic4.jpg" width="100px" height="100px" onclick="doIt(this)">

<img id="bigpic" src="blank.jpg">