Music changes the way we perceive any given situation in media. We add emotion and layers to the message that is being conveyed. The most interesting part of this relationship between music and media is that the media affects the inspiration for the music just as much, if not MORE than the music's impact on the chosen media.
One of the most enjoyable compositions I've written has actually been for my fiction novel "Civil". The above book cover inspires a lot of different styles of music: sci-fi, rock, drums, futuristic, action, and much more. It personifies the book very well, so whatever music the image itself stirs up is PERFECT for the book itself.
I came up with the Civil Theme and have been working on it ever since. I love taking a look at something, feeling the inspiration, and sitting at a piano to bring music to the scene before me. It's a great exercise and I suggest doing it yourself, whether it's a book cover, video, excerpt of gameplay, commercial overdubbing, silent movie, or just a picture/slideshow.
It's all about inspiration, timing, and dedication when it comes to bringing the perfect idea to life.