I'm struggling with this sentence. What does 'stand for a portrait' mean?
As this is about images, I thought illustration would be an excellent method of explanation.
Here is someone who is photographed when they stand for a portrait:
By comparison, here is someone who is photographed when they sit for an artist:
So: the person is in the standing posture in an artists studio while having their portrait painted. It can also apply to having a photographic portrait made in a studio*.
*I'm having trouble explaining in any more detail the language elements here...