It can be done only when HTML code is allowed. You can see if it is allowed in the "Posting Rules" box, just under the Quick Reply area.
In general discussion html is allowed. In Nonsense it is not. I am not sure if there is a reason for this difference.
the code to do it is:
HTML Code:
<EMBED src="http://www.website.com/files/flashmove.swf" WIDTH=400 HEIGHT=400 >