option_a.rb

class Page < ActiveRecord::Base
  has_attached_file :image, sizes: {small: '10x10', large: '100x100'}
  has_attached_file :uploaded_image

  def safe_image_url
    if image
      image_url
    else
      'spinner.gif'
    end
  end
end

module AttachmentResizer
  def self.dispatch page
    DelayedJob.enqueue SomeJob.new(page.class, page.id) if page.uploaded_image?
  end

  def self.resize page
    page.image = open(page.uploaded_image_url)
    page.uploaded_image = nil
    page.save
  end
end

class SomeJob
  def initialize klass, id
    @klass, @id = klass, id
  end

  def perform
    AttachmentResizer.resize @klass.find(@id)
  end
end

class PagesController < ApplicationController
  def create
    @page = Page.new params[:page]
    @page.save
    AttachmentResizer.dispatch(@page)

    redirect_to page_path(@page)
  end
end

# app/views/page.erb
<%= image_tag page.safe_image_url %>