Template edit if you want - This is how I do mine.

Change in postbit / legacy

Code: 
<if condition="$show['avatar']">
<div class="smallfont">
&nbsp;<br /><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a>
</div>
</if>
To

Code: 

<div class="smallfont">&nbsp;<br /><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="<if condition="$show['avatar']">$post[avatarurl]<else />images/avatars/defaultavatar.png</if>" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></div>