[Phantasmagoric]

Got the solution (thanks to Lynne for her help!) if anyone else wants it for their site as well.

Create a suitable image and call it (for example) moddev.gif, upload to your Skins misc folder (do this for all Skins installed!).

In the postbit template find (do this for all Skins installed!):

<if condition="$show['user_installed']"><div style="margin-top: 10px;background:url($stylevar[imgdir_misc]/bginstalled.gif) left no-repeat" align="right"></if>

After add (change the forumid array to match the forums you have used in the mod system settings!):

<if condition="in_array($thread['forumid'], array(28,29,32,10,39,43,41,51)) && $thread['postuserid'] == $post['userid']">
    <div align="left"><img src="$stylevar[imgdir_misc]/moddev.gif" width="100" height="21" border="0" alt="" /></div>
</if>

It's not a perfect solution as the developer image doesn't sit right at the bottom like the installed image does. The code for the installed image is wrong as the close div is missing, but if you close it the image doesn't show anymore. To avoid any further errors in the code I have coded it the way I have, I'm no coder so there is probably a better solution. If anyone knows of one then please share with us all.