Gajim does not use full-jid to retrieve vCard in MUCs although it is available
In MUCs wildfire-users' avatar does not show up. They do however for ejabber-users because it reveals their vCard, even if the request is not send to the full JID.
So wildfire not showing the vCard in anonymous MUCs does make sense and it is certainly not worth changing wildfire to decrease its users' privacy for anonymous MUCs, just to get their avatars showing up in non-anonymous MUCs where the full JID is available anyway.