I've been doing a bit of searching around and can't seem to find what I'm after (so please feel free to post a link to the right place !)
I've got the Blog module installed and running, with multiple contributors. They will only ever be Administrators or Content Authors on the site, so I'm not fussed about new member registrations or anything like that.
I just did smth similar few days ago on http://www.moviedeek.com/. The template you need to override is PageComment_instance I think. In my case I just included the picture provided by WillR's facebook connect module, but it seems like you already decorated your Member class with a profile image.
Thanks for the post. I got round it in the end by adding a function in the BlogHolder controller. With the member class suitably decorated as per the above thread), this function performs a custom SQL query to look up using the ID of the blog post to get the poster name and their avatar (along with a couple of other bits of info I added).
I call it from the BlogSummary template include, so it works at BlogHolder and BlogTree level.
I don't know if it's the "right" solution... but it seems to work OK !