The line in your postbit is just a series of underscores followed by a line break.
_____________<br/>You can't wrap THAT around your avatars.
But, if you search your postbit and/or postbit_legacy template for this code;
<if condition="$show['avatar']">
<div class="smallfont">
<br /><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a>
</div>
</if>
And replace it with something like this;
<if condition="$show['avatar']">
<div class="smallfont">
<div class="tborder"">
<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>
</div>
</if>
You may get something like what your after.
You'll just have to adjust the color of .tborder so that it does the whole "indent" thing, or create a new class and use it instead if you don't want to alter .tborder.
Hope this helps.