<<A quick Google confirmed my understanding that "head shot" is often used by, for example, actors and models to describe the glossy, well-produced "mug shots" they use to sell their wares.>>

These are often partial profiles.

<<Dear Seian: In computerese,"portrait" means longest side vertical, as contrasted with "landscape" meaning longest side horizontal.>>

Due to the different aspect ratios of the canvases ordinarilly used for paintings of these kinds, presumably.