I always thought they'd just be fairly expensive, like having an oil painting made in the real world...
Because I think that everyone and their brother would want to get a portrait made, and it takes a lot of time and effort to create a portrait.
Harry goes there after the battle and talks to DD, and I think all the other portraits are snoozing in their frames.
Considering that JK tells us that Harry makes sure Snape gets included, they almost certainly survived. Maybe they had other frames to flee to? Otherwise, there'd be no point to making a Snape portrait to add to the wall.
Actually here, have a quote from DH (The Flaw in the Plan):
But it was applause. All around the walls, the headmasters and headmistresses of Hogwarts were giving him a standing ovation; they waved their hats and in some cases their wigs, they reached through their frames to grip each other’s hands; they danced up and down on the chairs in which they had been painted; Dilys Derwent sobbed unashamedly; Dexter Fortescue was waving his ear-trumpet; and Phineas Nigellus called, in his high, reedy voice, “And let it be noted that Slytherin House played its part! Let our contribution not be forgotten!” But Harry had eyes only for the man who stood in the largest
portrait directly behind the headmaster’s chair. Tears were sliding down from behind the half-moon spectacles into the long silver
beard, and the pride and the gratitude emanating from him filled Harry with the same balm as phoenix song.
Some interview quotes that might be useful:
Something others mentioned before:
But this rule was always good, and the rule was that portraits could only move between portraits in the same building. so if I'm in a picture and you're in a picture and we're both in Carnegie Hall, then we can move into each other's pictures. Otherwise we can only move only to other places where we have a portrait. You can't just move willy nilly through all the - the Louvre, the Met - you can't do a world tour, as a picture person. You are limited by geography. So there was that reason. And then lastly of course, the third reason, is it really would be too easy and I wouldn't have had a plot.
Laura Trego: Was the absence of Snape's portrait in the headmasters office in the last scene innocent or deliberate?
J.K. Rowling: It was deliberate. Snape had effectively abandoned his post before dying, so he had not merited inclusion in these august circles. However, I like to think that Harry would be instrumental in ensuring that Snape's portrait would appear there in due course.
Q: Is Severus Snape's portrait in the headmaster's office?
JKR: Some have been asking why hasn't the portrait appeared immediately. It doesn't. The reason is that the perception in the castle itself and everyone who was in the castle, because Snape kept his secret so well was that he abandoned his post. So all the portraits you see in the headmaster's study are all headmasters and mistresses who died, it's like British royals. You only get good press if you die in office. Abdication is not acceptable, particularly if you marry and American. I'm kidding! [laughter] I digress. I know, because I thought this one through, because it was very important to me, I know Harry would have insisted that Snape's portrait was on that wall, right beside Dumbledore's.
This thing about "the perception in the castle itself" to me sounds like the headmaster/-mistress portraits come from within the Hogwarts castle. If a HM dies while he/she is in the post of HM, then from somewhere within the castle comes a portrait that has absorbed the essence of that person. Maybe it's something that the house elves do? Whereever it comes from, I don't feel like it'd be an outside commission – except possibly for Snape.