To create it I used one of my recent photos from Warsaw. What I did was quite simple but I won't share all details now as I would like to write a tutorial about it if there is enough interest. Generally speaking I blended together a sketch and a photo of the same place (sketch was created digitally as I'm not skilful with drawing...).
Also make sure to give my free eBook a try. You will find more than 20 pages full of photos in it.