There are two ways I know of to do this.
You can connect your device with Viber on it to a PC via USB cable. Once connected make sure to allow or give permission for file transfer from the mobile. On the computer open File Explorer. Browse to the following directory :-
This PC\<Your Mobile Device Name>\Internal storage\viber\media\Viber Images
From there you can copy media back and forth between your mobile device and the computer.
There is a Viber version that can be installed on you PC and the messages will sync with your mobile phone automatically. You can right click on the pictures from Viber on your PC, and it gives you the option to copy the Image, or open the file in the local computer folder.