[ozzy47]

This is untested however..

Edit your "postbit" or "postbit_legacy" template..
Find these lines:

<vb:if condition="$post.avatarurl">
	<img src="{vb:raw post.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />
<vb:else />
	<img src="{vb:stylevar imgdir_misc}/unknown.gif" />
</vb:if>

And change it to this:

<vb:if condition="$post['usergroupid'] == 1">
	<img src="http://something/guestavatar.jpg" />
<vb:else />
	<vb:if condition="$post.avatarurl">
		<img src="{vb:raw post.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />
	<vb:else />
		<img src="{vb:stylevar imgdir_misc}/unknown.gif" />
	</vb:if>
</vb:if>