You should learn to eject your hard drive by using the “Safely Remove Hardware tool” in the future.
But, now, in your case, you should not format this drive, if you still want to get back your pictures completely.
Stop saving new picture or other file on this drive at first.
Select a third-party recovery tool to restore your pictures back. In fact, there are plenty of data recovery tools out there. You should pay much attention to select a right one.
You could try some data recovery freeware: http://www.piriform.com/recuvahttp://freeware-fix.blogspot.com/2013/03/hard-drive-not-formatted-error.html
At last, save your recovered pictures on a different drive in case of data loss.
In the future, you should use your drive rightly and safely.