When a user logs in with a Twitter URL for the pic profile, it is set in the Users collection in services.twitter.profile_image_url and services.twitter.profile_image_url_https , so you can just read these URLs.
I use the following line to set the twitter user profile profile in the profile object, which is displayed to the client by default:
Meteor.users.update({_id:Meteor.userId()}, {$set {"profile.twitterpic":user.services.twitter.profile_image_url}});
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