Do you have Inkscape installed? I think it would be easier to draw the mask, i.e. the edgy edges, in there. You could just trace the one from your example pic as well. And then you could go back to GIMP and attempt the same procedure again. I would just make sure you convert all your images to PNG or JPEG. And to make sure you add the alpha channel before you do anything else.
Tbh I would use the clip function in Inkscape and then export the image as PNG to use in your slide. But I've always been more comfortable on the vector side of things.
The short answer is a court of law.
The long answer includes a reference to the location because a few countries do not list "escape from prison" as a crime in itself recognizing the human yearning to be free. So only incidental stuff would be interesting in a subsequent legal case, i.e. damage of property, threatening people with violence, etc. If you can manage to slip out in a laundry basket, you are okay. Andy Duphresne would be liable for the wall and sewage pipe he broke. (And committing fraud, of course.)