To give you a start (I am not a graphics person myself, I have to admit):

Have a look at QPainterPath. You might be able to do what you want by defining a QPainterPath in the shape you like, and then (once the user has clicked on the canvas) you calculate how the painterpath has to be transformed to get the result you like. (The transformation can be done by QMatrix/QTransform(in Qt 4.3)

You should also have a look at the Graphics View Framework which seems to be ideal for the task you plan.

Good luck!