How can I resize the Facebook avatar provided by OmniAuth using Gravatar?

Question

I'm able to get the Facebook avatar through Omniauth, but I'm only able to resize the default image not the Facebook image which is 50 x 50px by default. How can I resize it to 38px?

Here's my application helper:

module ApplicationHelper
    def avatar_url(current_user)
        if current_user.avatar.present?
            current_user.avatar
        else
            gravatar_id = Digest::MD5.hexdigest(current_user.email.downcase)
            "http://gravatar.com/avatar/#{gravatar_id}.png?s=28&d=mm"
        end
    end
end

and my User class:

class User < ActiveRecord::Base
    def self.from_omniauth(auth)
        where(auth.slice(:sprovider, :uid)).first_or_create do |user|
            user.sprovider = auth.provider
            user.uid = auth.uid
            user.first_name = auth.info.first_name
            user.last_name = auth.info.last_name
            user.email = auth.info.email
            user.cell_phone = auth.info.cell_phone
            user.avatar = auth.info.image
        end
    end