[Marris]

Well, I've got a semi-working popup in IE with ggiersdorf's code. This is the most consistent for me, but still not perfect:

 $navigator_user_agent = ( isset( $_SERVER['HTTP_USER_AGENT'] ) ) ? strtolower( $_SERVER['HTTP_USER_AGENT'] ) : ''; 
if (stristr($navigator_user_agent, "msie")) { 
    return '<img src="' .  $link . '" border="0" onload="if(this.width >= 400) {this.alt=\'Click here to see the orignal image\';}" style="width: expression(this.width > 400 ? 400 : true);" onmouseover="if(this.width >= 400) {this.style.cursor=\'pointer\';}" onclick="if(this.width >= 400) window.open(\'' . $link . '\',\'Image\',\'toolbar=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=yes\');"/>'; 
} else { 
    return '<img src="' . $link . '" onload="if(largerThan(this.width,400)) {this.width=400;this.alt=\'Click here to see the orignal image\';}" onmouseover="if(this.alt) this.style.cursor=\'pointer\';" onclick="if(this.alt) window.open(\'' . $link . '\');" border="0" />';
}

Replace all occurances of 400 with your desired width value.

In the IE code, instead of if(this.width >= 400), you are welcome to try combinations of these other if statements:

if(largerThan(this.width,400))
if(this.width > 400)
if(this.alt)

But neither of those worked consistently for me. I tried many different combinations and the code block above is what works best for me across IE, Firefox and Opera.

I welcome improvements.