To make everyone else's 50% try this one
PHP Code:
<vb:if condition="$show['avatar']">
<a class="postuseravatar" href="{vb:link member, {vb:raw post}}" title="{vb:rawphrase {vb:raw post['onlinestatusphrase']}, {vb:raw post.username}}">
<img style="width:<vb:if condition="$post[postcount] == 1">1</vb:if>50%" src="{vb:raw post.avatarurl}" alt="{vb:rawphrase xs_avatar, {vb:raw post.username}}" title="{vb:rawphrase xs_avatar, {vb:raw post.username}}" />