I don't know why this happens, but I have a couple of sims with transparent "icon" pictures. One of them is Buddy Bailey. In another game, Tim Burre had the transparent picture. It seems that it happens when the sim is a guy who wears glasses in Twinbrook! At least, in the ones I have seen. (maybe there's other guys with glasses who don't get the transparent picture)
What I see happening is: when you highlight the sim in the relationship bar, their face is about 20% transparent. Not totally clear, there's a definite outline and facial features are defined too. The hair is opaque, the neck/shoulders/shirt are opaque. When it's the playable sim, same thing when you see their face on the needs bar.
Anyone else have this happen?