See script on my GitHub:
https://github.com/panticz/magento/blob/master/scripts/mk_sitemap_images.php
\n";
    echo "\n";
}
function print_url($url, $date) {
    echo "\n";
    echo "    $url \n";
    echo "    $date \n";
    echo "    monthly \n";
    echo "    0.3 \n";
    echo " \n";
}
function print_foot() {
    echo " \n";
}
try {
    print_head();
    $collection = Mage::getModel('catalog/product')
        ->getCollection()
        ->addAttributeToFilter('image', array('notnull' => '', 'neq' => 'no_selection'))
        ->setStoreId(1);
    foreach($collection as $product) {
        // get last product update
        $updatedAt = substr($product->getUpdatedAt(), 0, 10);
        // get main image
        $image = Mage::getModel('catalog/product_media_config')->getMediaUrl($product->getImage());
        print_url($image, $updatedAt);
        // get additional images
        $gallery = Mage::getModel('catalog/product')->load($product->getId())->getMediaGalleryImages();
        foreach ($gallery as $image) {
            print_url($image->getUrl(), $updatedAt);
        }
    }
    print_foot();
} catch (Exception $e) {
    Mage::printException($e);
}
?>
>