Gravatars
Thanks to Michael Hampton’s comment, I now know that the Gravatar plugin I installed is working (I had to wait because I don’t actually have a gravatar).
A Gravatar is a ‘globally recognised avatar’ – you upload an 80×80 image to their site, give it a rating (using the American G, R, NC-17 etc.) and then any site which supports gravatars will display that image next to your name when you submit a comment. To comply with the general rating of the content on this site (and the ICRA label) I’ve restricted this to G-rated gravatars only (no R or NC-17, sorry).