Photos don't show after restoring WhatsApp backup

I had to reinstall Whats App in my iPhone 7, iOS 11.0.2.

I had a backup of my Whats App messages (plus photos and videos) in iCloud, so after installing Whats App performed a restore from that backup.

I have noticed that all the photos , in every chat, have disappeared (instead , a White box appears). However, the videos have restored perfectly well.