either you need to have a protocol implementation in your Clien-Server communication or you have to make sure that you send all the buffer at one time. If you are choosing a protocol means you first send a message to the server that am going to send a image buffer with size/length X and at the server store this and use this value for the next receive command.
If you are going to use whole buffer at one time, just use that buffer as such to create re-create your image. having a protocol is the best choice. if you google you can find many protocols choose the simplest one that suits your requirement.