hey. Sorry for such late comment.. I am not sure about a tutorial on this, but here is how i did it.. You can get an image or even a more graphical icon of imac or any monitor screen from Google. Just make guides on screen edges for one time. And whenever you need, just copy paste a wallpaper on it & resize with 'Snap to Guides' options on. Just the wallpaper have to be in 16:9 ratio to fit perfectly, or else you would to crop some part.