Why don't you use HTML to control and images HEIGHT and WIDTH properties for a thumbnail. And you can use JavaScript to open it in new page and view it in that page with the image an original size.
Just change to fit your needs.
<SCRIPT LANGUAGE="JavaScript">
function openIMG(){
windowOpen=window.open("",'Open','toolbar=no,location=no,directories=no,menubar=no,scrollbars=no,resizable=1,width=420,height =420');
windowOpen.document.writeln("<head><title>Image</title></head><body>");
windowOpen.document.writeln('<img src=http://www.mysite.com/mygif.gif border=1>');
windowOpen.document.writeln("</body></html>");
}
</SCRIPT>
And in the image:
<a href="javascript:openIMG();">
You can always assign openIMG(MyIMG) like so you can use it over and over again.
You basically get the idea, I hope.