Edit template: postbit_attachmentimage
Replace entire contents with:
Code:
<smallfont>$post[username] has attached this image:<p>
<center><a href="attachment.php?s=$session[sessionhash]&postid=$post[postid]"><img width="800" src="attachment.php?s=$session[sessionhash]&postid=$post[postid]" border="0" alt="Click to open the image in a new window" vspace="8" hspace="4"></smallfont></center></p>
Screenshot: (the mouse pointer is invisible on the screenie, but it was just next to the text in the yellow box)...