How do i implement an image which all connected players can draw on?

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Hello,
I'm currently working on a small multiplayer game. As a part of this game the players should be able to draw on a kind of "chalkboard". Every player should then be able to see/erase/draw everything on that board.
As this is my first ever game I don't really know how to update the image to all connected players... I got a working chalkboard an the code so that every player can draw on it, but one can only see what he himself has drawn...

I hope you can understand my problem and can somehow help me solve it...