HowTo - Image Style's 404 image How could I define an image for those images that are not found (404's). In other words, if image which is displaying with image style is not found, provide an another image?

If someone is interested here is my solution:

function themeName_image_style($variables) {
  // Determine the dimensions of the styled image.

  $dimensions = array(
    'width' =&gt; $variables['width'],
    'height' =&gt; $variables['height'],
);

  image_style_transform_dimensions($variables['style_name'], $dimensions);

  $variables['width'] = $dimensions['width'];
  $variables['height'] = $dimensions['height'];

  // Determine the url for the styled image.
  $variables['path'] = image_style_url($variables['style_name'], $variables['path']);

  // Load default image for 404's
  if(!file_exists($variables['path'])) {
    $variables['path'] = file_create_url('/sites/all/themes/custom/lsa/css/images/404.png');
  }

  return theme('image', $variables);
}
Go to Administration -> Structure -> Content Types, select the name of your content type, then click on Manage Fields.  Find the image field, click edit for that field, then scroll down until you see the Image Field Settings section.  Click the default image button, and select what image you want to show by default.