The default user avatar in Sakai (the graduation-cap silhouette) feels a bit dated, and also doesn't provide any differentiation in the UI for users who haven't updated their avatars. This makes things like Roster, Forums, Chat Room, etc. feel very static.
Let's follow Google's approach here, and create default avatars for every user in the system. This default avatar would be composed of a procedurally generated color background (w/ AA color accessibility for a white foreground), and include white initials for first / last name. See how Google does it:
As the default, this would then get overwritten by any photo uploaded by the user. It would also be exempt from the default profile photo permissioning, since it's the default photo.