I did a bit of experimenting. Here's what's happened so far.
"1) After updating your profile pic, go back to your forum profile page, then press F5. Your profile pic should now be up to date on your screen. This is how to test that the file upload worked and to make sure that the picture meets the forums technical requirements."
This worked perfectly, just as you described.
"2) Even if you have updated the profile picture correctly, old threads with your previous posts will likely initially display your previous profile picture be default. Press F5 if you see this, and your picture should update."
Yes, old avatar still shows but pressing F5 doesn't change it.
I'll wait a couple of days and see what happens.
Thanks for the ideas.