Mario Paint works with the Super NES mouse, and using the power of emulation you can use your computer mouse.
Once the game loads, press the ESC key to go to the Zsnes menu. Select config then add-ons. Check the mouse option and you will be good to go.
Export your artwork
You can bring your Mario Paint pictures to other programs by using the Print Screen key (Right of F12). Open your drawing program(MSPaint for example) and press ctrl+V, tadaa!