/**
 * Gives an image tag for an asset
 * 
 * @param sfAsset $asset
 * @param string $thumbnail_type
 * @param bool $file_system
 * @param array $options
 * @return string
 */
function asset_image_tag($asset, $thumbnail_type = 'full', $options = array(), $relative_path = null)
{
    $options = array_merge(array('alt' => $asset->getDescription() . ' ' . $asset->getCopyright(), 'title' => $asset->getDescription() . ' ' . $asset->getCopyright()), $options);
    $src = $asset->getImageSrc($thumbnail_type, $relative_path);
    return image_tag($src, $options);
}