Well, atleast if anyone else is interested, you can do it by making an actor and placing it in the center of the world. You will only have to make sure it doesn't interrupt with collision system and is drawn PRIOR to everything else. Altough, I'm open to any kind of suggestion if there is a better way to do it.

You could save the current background image of the world in a field (as a new GreenfootImage) and then draw your image onto the background. Then to remove, just set the saved image as the background image of the world. The following is like for within an Actor subclass: Hope this helps.

Well, as far as I've tried it, I've only got greenfoot to paint it on the background BELOW all the actors, so only the background gets the shading. Making the shadowmap an object and setting it a top draw priority does the effect just fine.

I believe I was mislead by 'drawn PRIOR to everything else', which would place it under the other objects, not over. As an actor overtop of everything, the only thing that will be affected is what actor mouse actions occur on.