_form.html.erb

<%# add this field within the form: %>

  <div class='field'>
    <%= f.label :image -%>
    <%= render :partial => "/shared/admin/image_picker", :locals => {
      :f => f,
      :field => :image_id,
      :image => @engine.image, # obviously change the variable for your purposes
      :toggle_image_display => false
    } %>
  </div>

<%# this should be all you have to do %>

db_migration.rb

# $: rails g migration AddImageIdToPages image_id:integer
# produces:
class AddImageIdToPages < ActiveRecord::Migration
  def self.up
    add_column :pages, :image_id, :integer
  end

  def self.down
    remove_column :pages, :image_id
  end
end

image.rb

class Image < ActiveRecord::Base

#...

has_many :pages

#...

end

page.rb

class Page < ActiveRecord::Base

#...
  
  belongs_to :image

#...

end