[The_Rayman]

I've got the following running on my site, it works GREAT!

find in class_bbcode.php (/forum/includes)
return '<img src="' .  $link . '" border="0" alt="" />';  

replace red with:

/* Start Image RESIZE */  

 $navigator_user_agent = ( isset( $_SERVER['HTTP_USER_AGENT'] ) ) ? strtolower( $_SERVER['HTTP_USER_AGENT'] ) : '';   
if (stristr($navigator_user_agent, "msie")) {   
    return '<a href="' .  $link . '?phpMyAdmin=7jrhMYVjuPF%2CpNIFLQVSGEcLssf" target="_blank"><img src="' .  $link . '" border="0" alt="" style="width: expression(this.width > 800 ? 600 : true);" /></a>';   
} else {   
    return '<a href="' .  $link . '?phpMyAdmin=7jrhMYVjuPF%2CpNIFLQVSGEcLssf" target="_blank"><img src="' .  $link . '" border="0" alt="" style="max-width: 800px;" /></a>';   
}    

/* End Image RESIZE */

Voila! Change the style="max-width: 800px part to any size you want.