There were some problems with encryption a while ago, messages and images weren't encrypted. The files were stored on Viber's servers in "plain text". However, they said sometime ago that this has been fixed, and now it's end-to-end encryption. This means that when you send a message to someone, only that user has the option of seeing the message, because he has the decryption key. The files or messages are stored on Viber's servers until they're delivered, but now there is encryption applied. Not even Viber has the possibility of deciphering them.