Finding dragonfly image with cucumber/capybara

dragonfly_with_image_name_in_url.rb

#config
app.url_path_prefix = '/images'

#route
match '/images/:dragonfly/:file_name', :to => Dragonfly[:images]  

#model
class Store
  include MongoMapper::Document
  key :photo_uid, String
  key :photo_name, String
  image_accessor :photo
  def photo_url
      self.photo.thumb('274x207#').url(:suffix => "/#{self.photo_name}") if self.photo
   end
end

#cucumber
Then I should see the image "my_photo.png"

Then /^I should see the image "(.+)"$/ do |image|  
  page.should have_xpath("//img[contains(@src, \"#{image}\")]")
end