I attached a snippet of your cover, that shows the image border, I´m curious about. I´d like to create a similar technique, in order to embed one image into another with different kinds of transitions.
Sorry, can´t explain it better, since my English reaches its limits when it comes to technical terms :)
So the most straight forward tutorial I found is for Photoshop, but the principles are exactly the same (Much of it is Photoshop specific and more involved than you really need, so just take away what clicks for you). You add a layer mask to one of the photos you want to combine. A white mask shows the image, while black hides it. You then paint on the mask (not the image) either white to expose, or black to hide what you do and don't want to see. Since you are not touching the actual image, you can fix or change whatever you like by repainting the mask as you need.
In the above case I used a very jagged brush and added incremental jitter (randomizes size and position of brush slightly to add variation). I positioned one image where I wanted it roughly on top of the other, and then painted the layer mask to suit the background.
Every separate part of an image should be a separate layer, not hard pasted onto the image so you can still manipulate it. Probably goes without saying, but just in case ;)
Photoshop video: https://youtu.be/ErDuKDNhYsI
Hope that helps...