indisplay look for
$showattach = qq~<center><img src='$

just look for $msf to see what 1 your useing  src='$upload_dir/$msf or  src='$msf

use this
$showattach = qq~<center><img src='$upload_dir/$msf' border='0' onload="javascript:if(this.width > screen.width-300)this.width = (screen.width-300)" onclick="javascript:window.open('$upload_dir/$msf','','scrollbars=1,toolbar=0,resizable=1,menubar=0,directories=0,status=0')" alt="Posted Image, might have been reduced in size. Click Image to view fullscreen." title="Posted Image, might have been reduced in size. Click Image to view fullscreen."></center>~;

or this not both
$showattach = qq~<center><img src='$msf' border='0' onload="javascript:if(this.width > screen.width-300)this.width = (screen.width-300)" onclick="javascript:window.open('$msf','','scrollbars=1,toolbar=0,resizable=1,menubar=0,directories=0,status=0')" alt="Posted Image, might have been reduced in size. Click Image to view fullscreen." title="Posted Image, might have been reduced in size. Click Image to view fullscreen."></center>~;


resizes images biger then 500 to 500 all others leaves alone...