grgrs, There are ways with scripts to make more advanced thanks buttons but the easy'est that i can make at this moment is with use of the reputation system.
go to CP --> Design --> Design management (templates) --> Forum --> Appereance of entries ---> Find:
place this before it.
<?if($DO_REP_URL$)?><a href="$DO_REP_URL$"><img onclick="repchange('plus','$ID$');" height="10" src="yourthanksbutton.png" border="0" /></a> <?endif?>
the only thing you need to do is make a thanks button in photoshop or any program that you can make it with