Here is a very contrived example showing DrawingCanvas's Save Image action to save the state of the canvas, then uses the On Saved Image trigger plus the Saved Image URL expression to set the saved image into an empty Sprite instance.
dropbox.com/s/0y7zm42smt7zhcw/DrawingCanvasSaveImage.c3p
For the case that you are describing, I think what you would need to do is keep track of the url of the saved image on the trigger, so you can retrieve it later on, outside of the trigger.