Its pretty much what was done in Dom2 to create the library we had. Someone also created a macro which worked its way thru the entire All_Units game to save all of the units to a graphic named for its unit ID number.
We do have a complete collection for Dom3. Altho its a couple of patches old so a few might be missing. Its in the Tools and Tutorials link.
Dom3 Unit Reference v3.04
If I remember correctly there were like 5 filters that could be turned off in order to get the cleanest image-grab. The result tended to look ugly without the smoothing and shadowing effects that the game does, but the end results were better. If you edit a "smoothed" copy of an image, and then the game smoothed it again, it tended to be pretty blurry.