image = Image.create(:id => 42, :filename => "foo.jpg")
image.id #=> 42

Dir.glob("/images/to/import/*.{jpg,png,gif}").each do |path|

  # simulate uploading the image
  tempfile = Tempfile.new(path)
  tempfile.set_encoding(Encoding::BINARY) if tempfile.respond_to?(:set_encoding)
  tempfile.binmode
  FileUtils.copy_file(path, tempfile.path)

  # create as you do in the controller - may need other metadata here
  image = Image.create({:uploaded_data => tempfile})
  unless image.save
    logger.info "Failed to save image #{path} in migration: #{image.errors.full_messages}"
  end

  tempfile.close!
end

class AddImages < ActiveRecord::Migration
  def self.up
    Image.destroy_all

    execute("ALTER TABLE images AUTO_INCREMENT = 1")

    image = Image.create(:filename => "foo.jpg")
    image.id #=> 1
  end

  def self.down
  end
end