How to make a button available to thread starter

[JohnBee]

I want to make a button with a function available only to the
thread starter and admins.

What condition would I use for this?

I tried:

     <if condition="$firstpostinfo[userid] == $bbuserinfo[userid]">
     image.gif
     </if>

But after someone else posts in the thread the image appears for them
and the original thread started no longer has the button