This install-and-done add-on will add user avatars to the left of any mentions of them.
This add-on uses the XF2 'embed metadata' system to store the mentioned users with each message. This means that for any message created after this add-on is installed, there will only be one additional query per thread for the user avatars.
For messages created
before this add-on is installed, there is an option to control what you wish to happen. This provides three alternatives: