Leverage browser caching gravatar image comments

is there possibility to cache this avatar appear in the comments? to speed up our website?
https://0.gravatar.com/avatar/054b84053ad0e02a0174bd89ccdf814a?s=60&d=wavatar&r=G (5 minutes)
how can i set expiration to this file with not hosted in my hosting?
please take a look here https://dl.dropbox.com/u/7588358/problemcomments.jpg