Chilton

STEP ONE:
Open up tumblr.com/customize and open "EDIT HTML"

STEP TWO:
copy/paste this anywhere AFTER "<style type="text/css">"

         #popup
         {right:0px; bottom:-3px; position: fixed; }

STEP THREE:
copy/paste this AFTER "<body>" but BEFORE "</body>"

        <div id="popup"><a href="http://scarletsword.tumblr.com/tagged/pop-up-gif"><img src="http://i.imgur.com/tG5Jxyd.gif"></a></div>