First, you'll probably want to get my bugfix patch since that enables you to set the folders to anywhere in the system. Leo has a copy(https://itch.io/t/667993/leos-mod) otherwise it is found on Strive's Discord #mods channel(https://itch.io/t/284398/discord). Enabling just this change can be done, the change log lists it for "imageselect.tscn".

For the patch to work with mods, you'll need to copy it over the game's files (Windows will merge the folders easily) and delete the "backup" folder. Then the game can be started and mods applied. Not all mods are compatible, random portraits should be fine.

Start or load a progress, select a slave, select "Customization", press either "Portrait" or "Full Body Image", and press "Select Folders". From there change both folders to the ones you want, though please note the difference between "Select This Folder" and "Select Current Folder".

Edit: added missing step.