The bottom line is:

Print anything that is untrue as long as it is followed as a disclaimer or exclusion to say that its just a joke, so in essence your comments on the avatar should not be based on truth.
Or if you use no disclaimer make sure you use something which is not a lie. Libel is something untrue, no liability if it is; It is a very grey area. Essentially your disclaimer needs to be big print and stated several times, the website your opening may be contravening privacy under the european convention on human rights.

Without such a disclaimer printing untrue comments may lead to libel action but from the person who can claim directly to have suffered the damage. The onus is on the claimant to prove it. Just because someone may be unknown does not mean they cannot suffer damage, but the likely of successful action is much weaker.
Some people may unwittingly become liable for breach of confidence, it may work for delicate intimate information ,evidential issues do arise here.

All what i am saying is the worst case scenario, but its not impossible.