I'm half asleep and the original portrait shots are rather dark to me. (I can't be bothered to open them in a detailed view bc of what I know.)
So gee whiz, WHOA FOLKS.... I'm pretty sure that the portrait between the season and moon phase has appearance changes based on the sim's moods at different times, and I have seen that regularly. Get your sim singed, hungry, sick, or other definite mood changers and look again. When the mood meter near the moon icon changes drastically, you see a greater degree of change. But when the sim is in a good mood, the portrait is not necessarily static.
It's not supposed to be completely static in my experience.
Edited to Add: see
Wiki-
In The Sims 3, if a Sim's mood is in the top section of the bar, they will automatically generate lifetime happiness points. Sims also learn skills and get promotions faster when they are in a good mood. The Sim's facial expression on its portrait changes with the mood.