Add avatars for comments

For avatars you could use Gravatar - its a service which associates avatar image with an email address. We have user emails, so we can use this Gravatar API: http://en.gravatar.com/site/implement/images/python/. Default avatar is not provided by Gravatar.
E.g. for albertas.gimbutas@mii.vu.lt email, the avatar link is http://www.gravatar.com/avatar/7942d2b4692b34c41890a6eb2fbeb819
