I think the *.spr-format has to be considered "proprietary", actually it is a sort of "container" for the images the unit animation requires (various directions, various kinds of movement etc. ...) and the "animation rules".
If you wish to add sprites to the game you basically need to create a set of TIFFs. There's a utility for merging this set of images into a *.spr-file: MakeSprite. This utility also contains sample "scripts" for adding standard rules to the different kinds of sprites (units, goods, or cities) -- so you usually don't need to worry about those rules. You will find a couple of guides for sprite creation here (I recommend to read the part about pure black and the "alpha channel" carefully).
Creating sprites for units like ships, airplanes, or tanks without "full" animation is fairly easy. Creating full animations for "walking" (infantry etc.) units is much more difficult -- you will see what I mean if you take a look at the current unit sprite list