Store the processed image on the datastore before being served by dragonfly (pre-caching). The url needs to be accesses programatically, like in a background job.

initializers_dragonfly.rb

# store the job before it has a chance to be served by dragonfly
images.configure do |c|
  c.define_url do |app, job, opts|
    thumb = ThumbTracker.where(job: job.serialize).first
    unless thumb
      uid = job.store
      thumb = ThumbTracker.create!(
                                   :uid => uid,
                                   :job => job.serialize
                                   )
    end
    n = ['', 0, 1, 2].sample 
    app.datastore.url_for(thumb.uid, host: "cdn#{n}.s3.aws.com")
  end

end