title: Paperclip set default images for all different styles
body: <p>So in my model I have different sizes for my avatar:</p>
  
  <pre><code>has_attached_file :avatar,
    :styles =&gt; {
      :thumb =&gt; "60x60&gt;",
      :small  =&gt; "80x80&gt;",
      :medium =&gt; "140x140&gt;",
      :large =&gt;   "300x300&gt;"},
    :default_url =&gt; "/images/default_avatar.png"
  </code></pre>
  
  <p>Now when I call <code>model_instance.avatar.url(:small)</code> it will return "/images/default_avatar.png" if there is no avatar. It will also return the same result for any style I choose.</p>
  
  <p>I want to make the default images change upon change of style so that when I call <code>model_instance.avatar.url(:small)</code> it should return "/images/default_avatar_small.png" and if I call <code>model_instance.avatar.url(:large)</code> it should return "/images/default_avatar_large.png"</p>
  
  <p>How can I make paperclip assign different default avatar for each style?</p>