We see this quite a bit and thought it would be worthwhile to add an FAQ on the matter.
We pull everyone's avatars straight off of GitHub's CDN infrastructure. We've noticed that sometimes it can take a while, even a couple of days for that change to both propagate across all of GitHub's various CDNs and get out of browser caches as well.
Rest assured, it will in time update. Unfortunately there's very little we can do about this.