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[]
WinTex[]
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.