EDIT

/**

 * @todo Please document this function.

 * @see http://drupal.org/node/1354

 */

function theme_cdm_preferredImage($variables) {

  $media = $variables['media'];

  $defaultRepresentationPart = $variables['defaultRepresentationPart'];

  $imageMaxExtend = $variables['imageMaxExtend'];

  $parameters = $variables['parameters'];

  $out = '';

  if (isset($media[0])) {

    $representationPart = $media[0]->representations[0]->parts[0];

    if ($parameters) {

      $representationPart->uri . $parameters;

    }

  }

  else {

    $representationPart = $defaultRepresentationPart;

  }

  $attributes = array(

    'alt' => (isset($media->representations[0]->parts[0]->uri) ? $media->representations[0]->parts[0]->uri : "no image available"),

  );

  $out .= theme('cdm_media_gallerie_image', array(

    'mediaRepresentationPart' => $representationPart,

    'maxExtend' => $imageMaxExtend,

    'addPassePartout' => FALSE,

    'attributes' => $attributes,

  ));

  return $out;

}