I just got the answer now only

Admin CP -> Styles & Templates -> Style Manager -> ? ? -> Postbit Templates -> postbit or postbit_legacy (depending on which layout you use)

For the postbit template, find this code and add the red code:



Code: 
<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 /><img src="URL OF BLANK AVATAR" alt="No Avatar" border="0" /></if>
Change the image URL appropriately.

for the postbit_legacy template, find this code and add the red code:

Code: 
<if condition="$show['avatar']">
				<div class="smallfont">
					&nbsp;<br /><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$avatarurl" alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a>
				</div>
			<else />
				<div class="smallfont">
					<img src="URL OF BLANK AVATAR" alt="No Avatar" border="0" />
				</div>
			</if>