That is downright weird, so I did a search, and someone in June had the same problem on "yahoo answers" and here is the answer straight from Viber themselves, hope it helps
"1. If a contact that is saved in your device's contact list has Viber or has had Viber at some time in the past, their Viber user photo will be saved to your device. This happens even if you have never communicated with this contact via Viber.
2. If you are part of a group conversation with users that are not saved in your contact list, their photos will also be saved to your device.
3. If any of the contacts mentioned above changes their Viber user photo, it will be saved to your device, even if they change the photo back.
For all of the reasons above, it is normal to see many photos in your Viber files, even if you do not recognize the photos or the people in them.
If you are concerned that none of the above apply to the photos you are seeing, please delete the photos in question (or even all of the photos in the file) and monitor the file to see if they reappear. If they *do* reappear, you should check the groups you are active in to see if this is a user that is not saved in your device contact list. If they do not reappear, it is likely that they are old user photos."
https://answers.yahoo.com/question/index?qid=20140608030909AAl9dUw
