They're third party. Look at the src URL of one. Here's muru's:
https://www.gravatar.com/avatar/0ce9def9898b5e9e0339ccc26c57ae94?s=48&d=identicon&r=PG
The jumble there is an MD5 hash of the user's email (failing back to network ID if no email set).
This can be overridden by the profile image. These are uploaded to Imgur (like other images here are) and takes preference if set.