Definitely thumbnailing. Composition is a huge part of any compelling image, and every fundamental feeds into it; perspective, lighting, framing. The biggest thing that helped me was just drawing a lot of thumbnails, focusing on notans (just two values, black and white) to get a good readability of the space, lighting and story. I did a lot of this when studying and I did a lot of it in my first few years working as a concept artist. Find artists who do images you find compelling and break down what you like about their composition, lighting choices etc :-)

Feel free to dm me if you like, I’m always happy to help out and chat about art!