maybe someone would like to correct me if i'm wrong but try this
inbetween your head tags:
CODE
<script>
function runit(w,h)
{
var ifr=document.getElementById("frame-name")
ifr.style.width=w;
ifr.style.height=h;
}
</script>
and with each thumbnail set what size the iframe has to be to accomodate the target image
<a href="image.jpg" target="frame-name" onclick="runit(100,100)">thumbnail1</a>
<a href="image.jpg" target="frame-name" onclick="runit(200,200)">thumbnail2</a>
<a href="image.jpg" target="frame-name" onclick="runit(300,300)">thumbnail3</a>