There is a "textures" folder in the mod folder. The mod will select a random image from this folder every time a providence is spawned.
If you're using r2modman, go to settings, then "Browse profile folder". Once you're in the profile folder, go into "BepInEx" then "plugins", then look for "CustomProvidence". The "textures" folder should be inside.
Manual Installation
Install BepInEx
Drag the CustomProvidence folder to BepInEx/plugins