add 6on9.com + extend imageshack.com

extend `imageshack.com/f/`: no need to download img first in order to view it.
test:
http://imageshack.com/f/0d20130417194727j
http://6on9.com/img/imgrgo.php?id=PeXq2vc.jpg
/f/ is a different location. one will not get the image to view directly:
https://www.google.de/search?hl=us&q=%22http%3A%2F%2Fimageshack.com%2Ff%2F%22&gws_rd=ssl