display avatar at certain post counts

[greatgamer]

how do i set up an avatar to show up when they hit a certain post count? for example if they hit 51 counts, the avatar becomes an alien, but when they hit 52, it returns to whatever custom avatar they had before? i have seen some boards do this but they stalwartly refuse to tell me their secrets lol. does anybody mind sharing their expert knowledge? i would really appreciate it