Variety of default profile pictures

[Semir1212]

Currently there is only one default profile picture which is used for all new users. Since many users do not upload a profile picture, it looks a bit clumsy.

We should add a bit more variety by e.g.:

• Adding more default images and assigning them randomly
• Adding more, and letting the users choose which ones they would like to use
• Generate lightly colored images with the users' initials, as is currently the case with spaces.