Discourse has recently begun showing ‘broken image’ thumbnails as placeholders from the tiny avatars in quoted posts.
Is some sort of rebuild needed?
In their support forum, this is the terminology they use when discussing how tiny Avatars are embedded: “the avatar is baked into the cooked post, so those posts will have the old avatar until/unless the post is rebaked.”
Other related terms: Gravatars, Identicons (based on IP or eMail, see Switch from Gravatar to HTML/CSS letters for no-avatar users - feature - Discourse Meta )