The image packs are put in the app data folder, also called user data, which can be accessed in-game by selecting a slave, pressing Customization, pressing either "Portrait" or "Full Body Image", and pressing "Open system folder". There is also a "Select Folders" button lower down if you want to change the paths to the image folders.
On Windows the shortcut to the app data folder is "%appdata%/Strive", which is equivalent to "C:\Users\USER_NAME\AppData\Roaming\Strive", with your user name inserted. The path can be opened using the Run utility, which can be opened with the Windows key + R, or through the start menu. Otherwise the path can be entered into the address bar of Windows Explorer.
Other problems:
If you run the game through the itch.io launcher then the app data folder will be in a different location, so usually it is recommended to download the zip archive for the game, extract it into a new folder, and run it as a stand alone program.
The game will not see the images if they are in a zip archive or if they are not tagged correctly. Many portrait packs use folders to tag the images, so removing the images from those folders will affect how the game uses them.