Should be possible. I imagine it would mean copying the button and using conditonals many times in the templates.
Look for this code (there are two instances of this in SHOWTHREAD):
Code:
<if condition="$show['newthreadlink']">
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]" rel="nofollow">
<img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" />
</a>
<else />
</if>
And you'll need to replace with this:
Code:
<if condition="$show['newthreadlink']">
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]" rel="nofollow">
<img src="$stylevar[imgdir_button]/
<if condition="$forumid == X">newthreadX.gif</if>
<if condition="$forumid == Y">newthreadY.gif</if>
<if condition="$forumid == Z">newthreadZ.gif</if>
<if condition="$forumid == A">newthreadA.gif</if>
<if condition="$forumid == B">newthreadB.gif</if>
<if condition="$forumid == C">newthreadC.gif<else />
newthread.gif</if>
" alt="$vbphrase[post_new_thread]" border="0" />
</a>
<else />
</if>
Where X Y Z etc are your forumid's. Your images should be in the button directory. The last one has an <else /> so that the regular button displays if you have not set one here. It's quite a hefty approach, and you would need to do this twice, but it works.
Hope this helps.