If you for example are creating a space game with a bunch of diferent star-sprites, it can sometimes be easy to lose the stars in the layout editor. wouldn't it be nice to be able to change the image that is displayed in the layout editor?
I'm pretty sure you already can. Under common there are options to change the icons used in the layout editor, specifically "icon" and "small icon" or something. Although you could be talking about something else, in which case that's worthless.