3D images for unit facing - how does it work?

[Armored Inferno]

I always play in 3D mode and the unit facing feature intrigues me from a programming standpoint. The /Specials folder in the West Front directory contains what might be called sprites -- an image file with 6 images in it representing the 6 facings a unit can have in a hex.
How does the game engine know how to find the correct image inside the master image of 6?
In web development, sprites have fixed X and Y coords within the sprite. Since these campaign series images have varying sizes, that doesn't seem like it could work in a formulaic way (example: if all master images were 20px by 120px, then each mini-image would be 20x20 and very easy to locate programmatically)