User profile pictures gets pulled from Gavatar if they have a gravatar account. I am not sure how often this pull happens, but there is a way to force download them
If you must, update it for everyone you can run:
./launcher enter app
rails r 'UserAvatar.find_each {|u| print "."; u.update_gravatar!}'
It will take a while to run
reference: Automatic updates of Gravatar avatars?