I am using last version of Viber on WindowsPhone 8.1. In Viber I can see member's thumbnail image, but I can't enlarge those images or view in full size. This is possible in Android version.
Try updating the software to the latest version, because I have the software installed on my Windows Phone 8.1 and I am able to tap on the member's images to view them in fullscreen. To be sure the images load, connect to the Internet and wait a little after you tap on a picture. Use the official website to learn more.