This simple plugin will allow you to set default profile picture for all users who didn't update their avatar yet.
phpFox has great feature – it can display user's initials with some color if user haven't set up his profile picture. However this doesn't work in all cases – sometimes admins want to set a pretty profile picture that will be assigned to all members who don't have profile pictures set up.
Default Avatar plugin solves this problem. Simply upload the image using AdminCP and it's working!