What breed(s) is the cat in this picture?

Hm. That's nothing like any cat I've ever seen. I'm assuming it's definitely a mix. Probably a simple black cat mixed with perhaps an Egyptian Mau or some sort of Bengal cat. They both can be gray and have odd stripes or spots all over their bodies. Though I'm not really sure. It could have just been one of the two and due to the quick movements and dim light you only saw a glimpse. That's nice artwork you've done there, by the way.