Ok :) Simply change forumhome_pmloggedin to this:
<br>
<smallfont><b>$bbuserinfo[username]'s Private Messages
<br>
<a href="private.php?s=$session[sessionhash]" title="Click Here to Open Your Private Messages $inboxname"><img src="pm_inbox.gif" align="absmiddle" border=0></a> <a href="private.php?s=$session[sessionhash]" title="Click Here to Open Your Private Messages $inboxname">Inbox <font color="ffcc00">($newpm[messages] New)</font></a>
<br>
<a href="private.php?s=$session[sessionhash]&folderid=-1" title="Click Here to Open Your Private Messages $outboxname"><img src="pm_sent.gif" align="absmiddle" border=0></a> <a href="private.php?s=$session[sessionhash]&folderid=-1" title="Click Here to Open Your Private Messages $outboxname">Outbox</a>
<br>
<a href="private2.php?s=$session[sessionhash]" title="Click Here to Open Your Private Message Tracking"><img src="pm_tracking.gif" align="absmiddle" border=0></a> <a href="private2.php?s=$session[sessionhash]" title="Click Here to Open Your Private Message Tracking">Message Tracking</a>
</b></smallfont>
<br>
As you can see, I've used images called pm_inbox.gif, pm_sent.gif, and pm_tracking.gif :)
There're some hardcoded colour codes in there you will probably want to change, btw :)