Paperclip doesn't give you image dimensions. I saw a bunch of different implementations and didn't like em. So I rolled my own. This implementation will save the `width` and `height` of images to the model in which you define your `has_attachmented_file` field.

migration.rb

# this migration allows to have polymorphic models
 
class CreateAssets < ActiveRecord::Migration
  def self.up
    create_table :assets, :force => true do |t|
      t.string   :type
      t.integer  :assetable_id
      t.string   :assetable_type
      t.string   :attachment_file_name
      t.string   :attachment_content_type
      t.integer  :attachment_file_size
      t.integer  :width
      t.integer  :height
      t.timestamps
    end
 
  end
 
  def self.down
    drop_table :assets
  end
end

paperclip.rb

# stick this inside the inialization directory or if you 
# append this to your Paperclip initialization file if you 
# have one already
 
module Paperclip
  class Attachment
    def image?(style = default_style)
     to_file(style).image?
    end
 
    def temp_file_location(style = default_style)
        to_file(style).path
    end
 
    alias :old_assign :assign
 
    def assign(uploaded_file)
      old_assign(uploaded_file)
      set_dimension_fields(uploaded_file.path)
    end
 
    def set_dimension_fields(path)
      return unless image? 
 
      dims = Paperclip::Geometry.from_file(File.open(path))
      instance.send("width=" , dims.width ) 
      instance.send("height=", dims.height)
    end 
  end
  
  module Upfile
    def image?
      ["image/jpeg", "image/tiff", "image/png", "image/gif", "image/bmp"].include?(content_type)
    end
  end
end