STIとポリモーフォック関連は相容れない ref: http://qiita.com/Naggi-Goishi/items/8dc6345c4324ae34817d

file0.rb

class User < ActiveRecord::Base
end

class Admin < User
  has_many :images, as: :imageable
end

class NormalUser < User
  has_many :images, as: :imageable
end

class Image < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
end

file1.rb

def open_image(user)
  user.image.open
end

open_image(admin_user)
open_image(normal_user)