sit for a/(one's) portrait
To pose while having one's portrait painted or photographed. It's tradition that every new president of the company has to sit for a portrait, which is hung in the main foyer of the building. With so many kids, it's always something of a nightmare trying to get everyone to sit for a family portrait each year.
sit for one's portrait
to serve as the subject of a portrait being done by a painter or photographer. I sat for the picture for two hours. Do you mind sitting for the painter all day? It will be easier if you get it over with all at once.