Random Avatar or Avatar by gender

How do I make users have a random avatar upon registration out of a group of avatars that the admin has predefined

for example say I have 50 predefined avatars, that users can choose from, is there a way of making the default avatar be random

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Or the other situation will be, out of the 50 above, 25 are female oriented and the other 25 are male oriented avatars. If after puting a mandatory profile field upon registration to select male/female

the code executes so that the male/female avatar is chosen according to the user's selected gender

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This however should all be ignored if the user then posts his/her own avatar

THANKS