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
why /f/ links exist then? if there are /i/ ones