picture.rb

class Picture < ActiveRecord::Base
  attr_accessible :picture, :picture_105px_width, :picture_120px_width
  
  has_many :pin
end

pin.rb

class Pin < ActiveRecord::Base
	belongs_to :picture_record, class_name: Picture
	def picture
		picture_record || read_attribute('picture')
	end
end

view.html.erb

<% if @pin.picture.respond_to?(:picture_105px_wide) && @pin.picture.picture_105px_wide.present? %>
  <%= image_tag @pin.picture.picture_105px_wide %>
<% else %>
  <%= image_tag @pin.picture %>
<% end %>