gistfile1.rb

class PromotionAttachment < ActiveRecord::Base
  belongs_to :promotion

  attr_accessible :like_gate_image, :promotion_image, :app_icon, :video_url

  validate :validate_like_gate_image_dimensions

  [:like_gate_image, :promotion_image].each do |file_name|
    options = {
      styles: {
        medium: "300x300>}",
        thumb: "100x100>"
      },
      :url => "/system/:attachment/:id/:style.:extension",
    }

    # If ENABLE_S3 returns TRUE (config/initializers/_tmg.rb) then files will be sent to s3 otherwise they will be stored locally
    options.update({
      storage: :s3,
      s3_credentials: "#{::Rails.root}/config/aws.yml"
    }) if ENABLE_S3

    has_attached_file file_name, options
  end

  protected
  def validate_like_gate_image_dimensions #i'd like to be able to check that a like_gate_image meets the right dimensions
    dimensions = Paperclip::Geometry.from_file(file.queued_for_write[:original].path)
    # why are you setting these?
    # self.width = dimensions.width
    # self.height = dimensions.height
    if dimensions.width != 810 && dimensions.height > 200
      errors.add(:like_gate_image,'width must be equal to 810px, and the height must be atleast 200px.')
    end
  end

end

This is much cleaner refactor. I just learned a lot. Thanks my G!