my understanding of muscle injuries is that if its swelling and within the first 24 hours of the injury, use ice. afterwards, use a heating pad only after the swelling goes down and to increase blood flow in order to start the healing process.
for a bruise like that and assuming that it stops hurting (no pain unless you get hit), i would think there are alot of broken blood vessels which probably need heat. also, massage therapy and stretching of the area would also speed up the healing process.
also dont stop training. getting alot thigh bruises is part of the growing pains in learning muay thai. learn how to maintain a more balance muay thai stance so you can switch faster between offense (ie attacking with your hands) and defense (ie defending with your legs).