Here's what would have to be added to the source, as with a graemlin (after creating a "spoiler.gif" image):

Code
< a href="javascript:void(0)" onclick="insertAtCaret(document.replier.message,'whatever text is chosen as spoiler space filler '); 
document.replier.message.focus();" ><img src="http://www.lcficmbs.com/ubb/graemlins/spoiler.gif" border="0" alt="Spoiler" /></a>  &nbsp;
malu