The game does not come with any images for random slaves. You need a portrait pack for that. You may also want to use the Improved Random Portraits mod by Zepetus, though you would need to use a different game files to enable mods.
The errors you see there are only related to the mod system. For MacOS the game files are distributed inside the .pck archive which prevents the files from being edited by the mod system. This post provides the stripped down version of the MacOS game that will work with the guide found in Help menu of Mods: https://itch.io/post/5418094