After several days I contacted the folks at Gravatar.com and this is what they said -

Hey there,

I've invalidated the image cache for you to see if that'll
help, but it's possible that a server between here and there
is caching the image because (and still hanging onto it) of
the .jpg extension you're using.

Are you in charge of that forum by chance, and if so, are
you able to change the image urls so they don't include .jpg
or .png extensions?

Velda | Happiness Engineer | Automattic

So what extension are they expecting?

Ralph

Hi Ralph, I think they prefer no extension on the name.