works in irb, not in rails

create_collage.rb

require 'RMagick'
require 'open-uri'
require 'ap'

image_urls = %w[http://images3.cpcache.com/product_zoom/215803453v6_460x460_Front_Color-Cardinal_padToSquare-true.jpg http://images2.cpcache.com/product_zoom/292564282v8_460x460_Front_Color-MilitaryGreen_padToSquare-true.jpg http://images0.cpcache.com/product_zoom/57297330v27_460x460_Front_Color-Black_padToSquare-true.jpg]
images = (0..2).collect do |index|
  begin
    blob = open(image_urls[index]).read
    image = Magick::Image.from_blob(blob)[0]
    image.resize_to_fit 200, 200
  rescue
    ap $!
  end
end