I have not looked much into godot's offering but there is web socket support.
In order to be secure with a login use a web socket via https. You can then establish a connection, where login credentials are sent first. You can keep the websocket connection open indefinitely and send data as you like.
The back end needs to be setup to handle the web socket connections and process information sent from your app, the first step being authentication. After that you exchange any quantity of information.
The image may need to be encoded, but you may be able to send byte data (it's been a while since i used websocezkts so.. it may need encoding into text)