I wanted to have a single avatar hosted on a web server. And I wanted to use it on each phpbb3 forum I install somewhere, simply adding my profile URL in my user settings.
I got it running with PHPBB3 RC7. It could have worked with previous versions, but only today I found the trick.
The trick is really simple. The avatar settings failed with the message "unable to obtain the remote image size".
And the message is rather self-explanatory : this meant that, probably for security reasons, the webhost refused the query asking for the image size. So, the trick is simple, remove the avatar size "0" (meaning automatic size) and give the real size you want for your avatar, for instance 100pixels*100pixels.
And it works.
So, now, I can proudly use the same avatar on each phpbb3 forum I subscribe. Isn't this great ?