if you have the game on steam there is a bunch of work shop items that have portrait packs, i think for when i create characters i have like 2000 portraits or so, in fact my game freezes for a couple of seconds when the portrait option comes up because i have so many to choose from ahaha
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There are lots of ready-made portrait packs on the Vault. The Custom Content Guide tells you how to make your own.
If you're talking about the OC, you need to copy it to a custom module first.
The hard graft is working through every area or template in the toolset, changing each NPC's portrait by hand.