title: Add image in pdf using Prawn
body: <p>I've a problem for adding images into a PDF using Prawn as pdf generator. I'm trying to add image using the following code:</p>
  
  <pre><code>def header
    text "something"
    image "#{Prawn::DATADIR}/images/logo_small.png"
  end
  </code></pre>
  
  <p>But app replies to me with the following error:</p>
  
  <pre><code> uninitialized constant Prawn::DATADIR
  </code></pre>
  
  <p>Where is the mistake?</p>
  
  <p>P.S. My app is running on Rails 3.1 &amp; Ruby 1.9.2.</p>