I don't think they all have to be in jokes, but try not to just take pictures of yourselves and tacking 1337 or popular phrases to it.
A rule I would use is: If the picture actually took some effort to take or is of something significant than it is a winner. This rule doesn't work if you have low standards concerning what an "effort" would be. Or if the "significance" of the picture only applies to you and 3 of your non-FF buddies.