[GameWizard]

I think it would better to use this:

<if condition="$show['member'] AND $show['avatar']">
<a href="member.php?$session[sessionurl]u=$post[userid]">
<img src="$post[avatarurl]" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" width="48" height="48" border="0" /></a>
<else />
<img src="defaultavatar.gif" alt="Avatar" width="48" height="48" border="0" />
</if>

Firstly the $show['member'] will apply to all members, regardless of usergroup, and then the <else /> will only apply to guests.

Btw, make sure you use <else /> and not</else>