I've searched for this issue, but it appears no one else is having or encountered it which means it's a localized error, therefore the approach needs to be a standard one. For example, backup your Viber folder with all its contents. You can simply use the built-in File manager to move the folder.
After this procedure, go to Settings > Applications > All tab > Viber. Tap on Viber and use Clear Data and Clear Cache. Reboot the phone, add the account again, initiate the chat with the user sending you images and then wait for results. Alternatively, you can add the folder back to its location, except the images. Only messages and user's images should be added.