If you want to see all the avatars / photos that your Viber contacts use for avatars (even old ones) then you can find big sized images here (this is only for Windows):
DRIVE:\Users\YOUR WINDOWS PROFILE\AppData\Roaming\ViberPC\YOUR PHONE NUMBER\Avatars
so for example if your drive is C, your windwos profiel is FooUser and your phon is 555-55555 then you can find it here:
C:\Users\FooUser\AppData\Roaming\ViberPC\55555555\Avatars
Note: The files might not have standard photo extensions (jpeg,png,.) so might not be able to open them with double clicking on them – but they are still photos. So just drop them into your favorite photo viewer or even better: browse that path with your photo viewer’s explorer.