# image.rb
class Image < ActiveRecord
  has_many :comments, :as => :commentable
end

# post.rb
class Post < ActiveRecord
  has_many :comments, :as => :commentable
end

# comment.rb
class Comment < ActiveRecord
  belongs_to :commentable, :polymorphic => true
end

# config/routes.rb
resources :posts do
  resources comments
end

resources :images do
  resources :comments
end