GM Techniques: What is a good cooperative drawing tool to use in an online game?

In offline games, I liked to use a piece of paper to draw things and use it as a simple battle map. Recently I started playing in Discord and I lack the option to draw something. What I am looking for is basically MS Paint for cooperative drawing. I am not interested in using some kind of grid system or a sophisticated drawing program. If the software worked on all platforms, the world would be ideal, but I would also settle for something that is just phone or Windows.