Kaito it's written in about 10 places on this forum.
Are you on Windows?
If so, go to the folder "%AppData%/Roaming/Strive". Hit the windows button, type that, press enter. If you can't figure that out, go to "C:\Users\USERNAME\AppData\Roaming\Strive".
In that folder, you'll find "portraits", "bodies", and "mods". Put the portraits in "portraits", bodies in "bodies", and mods in "mods".
For images, it may now find them automatically, or you can choose them by going Customize -> Choose image for each slave.
For mods, open the game, press "mods", activate the ones you want, then hit enter and it will exit the game. Now restart and TADA you have mods.