For that, you would have to alter the IMG tag parser. *might* be just a matter of editing bbcode.inc.php and convert the line
return sprintf('<img src="%s" %s alt="" />', $url, $align);
into
return sprintf('<a href="http://www.fccuk.org/forum/imagepopup.php?image=%s" onclick="window.open(\'http://www.EXAMPLE.COM/FORUM/imagepopup.php?image=%s\', \'imagepopup\', \'toolbar=no,scrollbars=no,width=300,height=300\'); return false;"><img src="%s" style="border: 1px solid black; padding: 5px;" height="60" vspace="5" alt="click to enlarge"></a>', $url, $align);
And upload the PHP script provided.
Am actually pretty sure you'll have to check out that "sprintf" thingy and take out the 'align' stuff AND/OR just make it so that images don't have the option of being floating. I'm sure someone else will jump in and give you a solution for this. I'm not going to try this out on my live board to be honest. For one thing, we don't allow inline images - only this image thumbnail thingy.
