Draw is a simple project to make a paint program to let you draw on the screen. Download draw.sb into Scratch and view the video to see how the application is made from Scratch.
Paint a picture with the mouse. Press the green button to clear the page, P to put the pen down and back up again, C to change colour, and the left and right arrows to change the width of the pen.
Here is a whole gallery and a couple of drawing projects on the Scratch website: