Adding custom sprites
The editable sprites have a four-letter code, a letter key, and one or two rotations. A good example of this is the Imp's first walking frame lumps, labeled TROOA1, TROOA2A8, TROOA3A7, TROOA4A6, and TROOA5. Rotation-wise, if a lump adds another rotation to its name, then the opposite rotation is a mirrored image of the same sprite image. See sprite for a more thorough discussion of Doom's rotation and animation system.