Even the subspecies still have a race, so any randomportraits (mod or built-in) function will try to assign according to race. Example, a Gnome subspecies Imp will get Gnome pictures assigned. My personal preference would be to manually assign a portrait/body of an imp for that character.
As for suggested pics, there are some great portrait packs out there and some of them will fit well (ie. there's some mermaids listed as Nereid). I've been working with JJoseph on the Discord to get together a new portrait pack from couple thousand images I had lying around and got from folks on the Discord. I'll start putting aside some specifically for subspecies that might be harder to match. Let me know if anyone has any requests though, and I'll find some in response.