Check out Mockup Scene Creator - Desk edition by Place.to on Creative Market - It's not free but it's a sweet piece of work. No doubt the result of countless hours and well worth the price if you're looking for desk mockups.
Posters Mock-Ups (PSD) These mock-ups are made for the presentation of the vertical oriented image. Also there are some extra elements that I’ll tell you about a bit later. You can use it as you wish, due to the license, of course.