title: Magento: Change default product image with PNG
body: <p>In order to replace default product image in Magento, we have to replace images in this folder:</p>
  
  <p><code>skin\frontend\mytheme\default\images\catalog\product\placeholder</code></p>
  
  <p>There are 3 files:</p>
  
  <ul>
  <li>image.jpg</li>
  <li>small_image.jpg</li>
  <li>thumbnail.jpg</li>
  </ul>
  
  <p>I need to replace those files with PNG. Where is the code that I can change the <code>.jpg</code> part?</p>