Aah, but (going back to my wee avatar problem) I
did click on the "Edit Avatar" link and it gave me this lovely message:
Your Current Avatar
No Avatar Specified
Avatars are small graphics that are displayed under your username whenever you post.
Do not use an avatar
Note: if you have a custom avatar selecting this option will delete it.
And that, as they say, is it. No box to put in my URL, no option of uploading, nothing.