image_proxy_controller.rb

require 'base64'
require 'net/http'

class ImageProxyController < ActionController::Base
  def get
    url = URI.parse(Base64.decode64(params[:url]))
    image = Net::HTTP.get_response(url)
    send_data image.body, type: image.content_type, disposition: 'inline'
  end
end

image_proxy_helper.rb

require 'base64'

module ImageProxyHelper
  def proxy_image_tag(source, options = nil
    options[:src] = "/image_proxy/#{Base64.encode64(source)}"
    tag("img", options)
  end
end

profile_controller.rb

require 'net/http'

class ProfileController < ActionController::Base
  def image
    if params['id'] == 'john'
      url = URI.parse('https://pbs.twimg.com/profile_images/1979976243/potato_300.jpg')
    elsif params['id'] == 'cgarvis'
      url = URI.parse('https://pbs.twimg.com/profile_images/1809634620/twitter-profile.png')
    end

    unless url == nil
      image = Net::HTTP.get_response(url)
      send_data image.body, type: image.content_type, disposition: 'inline'
    else
      send_file 'default_image_path.jpg'
    end
  end
end