I don't think I need to describe this one, as it's one of the more familiar MODs out there.
EDIT: I just remembered you can add such a field, but can the male or female images be added? If not, I'd like to request such a feature.
In addition to such a MOD, I was wondering if anyone has an idea of how to preserve the gender members have specified before making the upgrade to phpBB3, and then being able to install the gender MOD and then have those members who've already specified their gender before the upgrade have their gender show up again instead of having to respecify. In short. . .
-- keep info on who is what gender
-- upgrade and install gender MOD
-- apply gender of those who've already chosen one before upgrade.