This is the data source repo for this game: https://github.com/davidjoffe/dave_gnukem
NB This repo is NOT essential for the game to run
This repo contains the 'source files' used to generate the actual game data files. For example, it contains the original Photoshop files (with all original layers, styles etc.) that are then exported to e.g. TGA format (or in a future version we may use e.g. PNG) in the game's data folder (in repo https://github.com/davidjoffe/gnukem_data). The game itself only uses the exported file, and not e.g. Photoshop files.
This repo would thus only be necessary if you intend to literally do tasks like e.g. sprite editing in Photoshop. (You can also clone this anywhere, unlike the gnukem_data folder, it does not need to be in a particular location for the game to 'find it'.)
Thus if, say, editing sprites in this repo, the exported products would typically likely be intended to go to this repo, where the game data lives (unless you were creating a derived game):
https://github.com/davidjoffe/gnukem_data
See the above repo for license details