Rails has_many :through Polymorphic Association (http://goo.gl/lxmehk)

image.rb

# app/models/image.rb

class Image < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

post.rb

# app/models/post.rb

class Post < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

tag.rb

# app/models/tag.rb

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :posts, :through => :taggings, :source => :taggable,
    :source_type => 'Post'
  has_many :images, :through => :taggings, :source => :taggable,
    :source_type => 'Image'
end

tagging.rb

# app/models/tagging.rb

class Tagging < ActiveRecord::Base
  belongs_to :tag
  belongs_to :taggable, :polymorphic => true
end

I don't think a straightforward solution for this exists. Probably I'll need to change the database schema to account for this.

Nice work and thank you for posting. I'm curious, what's the tagging migration and schema look like?