Sprites in Doom are easy to edit with the right tool. Usually, they are exported/imported to and from .bmp images via DeuTex or its Windows counterpart WinTex. On the Macintosh port, graphics are edited on a slightly cruder interface via the Deimos program (the name standing for 'Doom Editing Interface for the Macintosh Operating System').
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.
Adding sprites to WADs
Adding sprites with WinTex is relatively simple. First open the WAD, and then select the Sprites section from the menu or by clicking the button with a green character. Sprites can now be loaded from existing bitmap files by choosing Load entry from file from the Edit menu. The dialog gives you a list of template names for some of the most common sprites. For example, it gives SARG@#@# for the demon, in which @#@# must be changed to the code that reflects the desired animation frame rotation.