Stub a image url and return an image

How can I stub a call to a url eg http://www.example.com/images/123.png and return an image named 123.png?

I'm using Rails 3.2, Carrierwave. I've tried Fakeweb but got a bit stumped.

After a few hours of research, turns out it's simple:

FakeWeb.register_uri(:get, string_or_regxp_of_uri,
        body: SupportFiles.uploaded_file("square.jpg"),
        content_type: 'image/jpg')

My problem is more tricky:
I am testing FB avatar, and I got whitelst extension

The above code will NOT work since the extension is missing
(FB avatar URL: https://graph.facebook.com/123/picture)

But the real FB avatar will redirect to a CDN or something that has the extension
So you need to add another stub:

# Register a fake remote image
fake_avatar_uri = "https://graph.facebook.com/fake_avatar.jpg"
# Redirect to a fake uri
FakeWeb.register_uri(:get, %r|https://graph\.facebook\.com/|,
  status: ["301", "Moved Permanently"],
  location: fake_avatar_uri)
# Feed fake image for the fake uri
FakeWeb.register_uri(:get, fake_avatar_uri,
  body: SupportFiles.uploaded_file("square.jpg"),
  content_type: 'image/jpg')

The SupportFiles module (not written by myself :P):

require 'rack/test/uploaded_file'

module SupportFiles
  extend ActiveSupport::Concern

  included do
    let(:an_image) do
      open_file("square.jpg")
    end
  end


  def open_file(filename)
    File.open(support_file_path(filename))
  end

  # idea stolen from ActionDispatch::TestProcess#fixture_file_upload
  def uploaded_file(filename)
    Rack::Test::UploadedFile.new(support_file_path(filename))
  end
  module_function :uploaded_file

  protected

  def support_file_path(filename)
    Rails.root.join("spec/support/files", filename)
  end
  module_function :support_file_path

end