So, the only slaves that automatically get a portrait are the special ones, all the generic ones. All the default ones don't get any portraits normally. There's an option to enable random portraits, but it requires portraits to have matching races in the names so you can't just dump any files into the appropriate folder.
The appropriate folders are:
C:\Users\<your_name>\AppData\Roaming\Strive\Portraits
C:\Users\<your_name>\AppData\Roaming\Strive\Bodies
If you download packs, or use your own images, they need to go in one of those 2 folders.
Once there are images in those folders, you can select a slave in your mansion, go to customization tab, and click portrait to select a new one. If nothing comes up in that, then either you got the path wrong, or you've potentially changed the default path. Without any specific details of what you've done though it's hard to guess.