Speaking of...
IMO, the main goal of a promo pic is to give folks an idea of what you look like and indicate the band/artists image. A promo pic should also be practical as well as artistic. A good promo pic should be reproducable across different mediums. What looks great on the web, could look horrible in print. What looks great at 2816x1880 pixels make look unintelligible at 125x125. And vice versa, what may looks crisp and sharp at 640x480 may look totally blurred when blown up to 1600x1200. Like a great recording is tested against a number of different speakers, a great image should be tested against a number of different mediums such as computer monitors, printouts, phones, etc. to make sure it looks good regardless of size, format, or medium.