How do I add my own textures?

Open the game and look for the configgy menu.
There should be an "Open Textures Folder" button. Click it to open the mod's appdata folder (generating upon starting the game with the mod installed).
Inside there is a "textures" folder. The mod will select a random image from this folder every time a providence is spawned.
There is also an optional "rare textures" folder. You can set the probability for images to be picked from that folder instead of the regular one in the Configgy menu.

Linux User Guide

If you're playing through Proton, the mod's appdata folder will be in steamapps/compatdata/1229490/pfx/drive_c/users/steamuser/Application Data/CustomProvidence