Like I wrote above, your code works as-is currently. To add a header to the data that you want to send over the socket, you first need to know its encoding. Assuming that it is UTF-8, you can do the following:
header = "Header!\n"
header_bytes = bytes(header, 'utf-8')
socketData.sendall(header_bytes + image_data)
bytes
type can only be concatenated with bytes
in Python 3.