Sending an image with protobuf

With my post I want to send an image, I read here that it is possible to send an image with bytes. So I converted my image to byte. Now iam is stuck with creating a form that's a bytestring array so I can use it. I was going to look into google documentation and found the ByteString class, so I tried calling the construct, but this is private. so is there any way to make my bytearray into a bytestring?