I would add
function clickedGlasses(glassesImage) {
document.getElementById("glasses_overlay").src = glassesImage;
document.getElementById("glasses_overlay").style.display = "block";
}
To your script tag at the top, and then
<img id="glasses_overlay" src="blank.png" style="display:none;">
Into your "glasses" div alongside the "faceGlasses" image (you'll need to position this over the top of the other div, either absolutely, or relatively)
And finally, change your "helm" img to
<img class="frame" src="images/helmStatic.jpg" id="helm" border="0" width="71" height="40" onmouseover="rollOver()" onmouseout="rollOut()" onclick="clickedGlasses('images/helmOverlay.png')" />