Not necessarily looking for exact code, although not opposed to it, but how would you go about making a program that loads a picture to the background and then covers said image with a strait fill color of say gray, and when the mouse hovers over the picture it uncovers the part of the picture under it with a square like eraser type tool.  As you move the mouse over the picture, the parts you have uncovered stay uncovered.  Sort of like in paint when your erasing something, only under all he gray is a picture that does not get erased with it.