During our testing We changed our avatar’s several times and each time the change took effect in Transifex. In one instance though it took a couple of days. I also tested the gravatar with other applications such as Jira. The changes in Transifex took the same amount of time as the changes in the other applications. The message in gravatar that we received was "
Please allow 5 to 10 minutes for avatar changes to take effect
(Have you cleared your cache?)
". This leads me to believe that the issue may reside in gravatar itself, since you stated that you cleared your cache. I would recommend that you try to contact Gravatar in order to see if there is something wrong with your account there. I would also remove the new picture from Gravatar and add it again.
Please let me know if this helps.