Dogwarts Movie Poster

I'm getting a lot of practice with my compositing ability on Photoshop. I'm helping someone make a Harry Potter themed yearbook as a gift for someone containing That persons dogs. I am definitely a beginner when it comes to compositing. I would love to be able to make movie poster looking Composites using my own photographs. Below is my first attempt ever. The photos weren't specifically shot with this Composition in mind, so the lighting isn't exactly right on them. But if anybody has any suggestions on People I should be following, or YouTube videos. I should watch to improve my compositing , I would be greatly appreciative.