From what I can see, by the time it reaches the Nabble page it's been converted to a PNG file.
Googling for an answer to the animated PNG issue, took me to:
http://www.squaregoldfish.co.uk/software/animated-png/which provided some background.
Personally, I'd feel that animated avatars just distract from the text in the post, but given the subject of my forums, all that motion would just make readers feel still more seasick! :-)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.