Try like this :
<script language=javascript type=text/javascript>
function check_value(){
switch(document.test.field.value){
case "one":
document.getElementById("imagetest").innerHTML = "<img src='images/firstimage.png'>";
break;
case "two":
document.getElementById("imagetest").innerHTML = "<img src='images/secondimage.png'>";
break;
case "three":
document.getElementById("imagetest").innerHTML = "<img src='images/thirdimage.png'>";
break;
}
}
</script>
<form name='bob'>
<input type="radio" name="field" value="one" onchange='check_value()'>one
<input type="radio" name="field" value="two" onchange='check_value()'>two
<input type="radio" name="field" value="three" onchange='check_value()'>three
</form>
<div id='imagetest'>
</div>