Autoresize Script Worked Well But Top Banner Also Resized

My community runs a set of forums, (phpbb with the Brushed Metal template, if that is important.) and people often use large images in their posts. This ends up cutting off the majority of the image, so we thought we'd install an auto-resize script, to resize anything wider than 600 px. It works too well, it also resizes the banner at the top of the screen. A bunch of us hacked at it trying to get it to work, but none of us know anything about javascript, so it's not going so well. Either the script still resizes everything, or it does nothing at all.

Here's the earliest version I could find. It's not the original script, however...
<script>
onload_functions.push('resizeimg();');
function resizeimg() {
if (document.getElementsByTagName) {
for (i=0; i<document.getElementsByTagName('img').length; i++) {
im = document.getElementsByTagName('img')[i];
if (im.source == '[URL]')
/*PATH TO TOP BANNER THAT SHOULD NOT BE RESIZED*/
{ continue; }
if (im.width > 600) {
im.style.width = '600px';
eval("pop" + String(i) + " = new Function("pop = window.open('" + im.src + "','phpbbegypt ','fullscale','width=400,height=400,scrollbars=1,resizable=1'); pop.focus();")");
eval("im.onclick = pop" + String(i) + ";");
if (document.all) im.style.cursor = 'hand';
if (!document.all) im.style.cursor = 'pointer';
im.title = 'Click Here To See Image Full Size ';
}}}}
</script>
We're stuck, we have no idea what to do.