Cool - after I replied, I looked at Jake's post and realized there was a bit of an omission. To make it look right, you'd need to find this in postbit:
<if condition="$show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$avatarurl" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>
And replace it with this:
<if condition="$show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$avatarurl" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td><else /><td class="alt2"><img src="Your Avatar URL" alt="No Avatar" border="0" /></a></td></if>
Jake's additions left out the table bits.