I need to create an avatar base for my users to choose from

Hello,
I am using buddypress. I can see that users do put their own photo on profiles. but several do only keep the basic default image. It would be nice if i could give my users a choice of my own list of avatars, all in the same design ( about 15 to 20) that users can choose from.
How can i achieve that ? whaere sould i put avatars for my suser to choose ?
Best rgs,
Hervé