If the painter bases a painting on a photograph but puts his or her mark on it by adding their own significant elements hence changing the nature of the work from the original photo it becomes a new work. No need to ask permission I believe.
I suppose it all depends on how significantly the original was changed, which is probably a very subjective discussion. Even after adding significant elements it might be quite obvious the painting was copied from an image.
I'll be the first to admit I'm no lawyer, but some very quick research on google seems to contradict this.
From US copyright law: "Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work." What constitutes a new version?
There are several sites for painters that discuss this and they seem to follow the stance that you shouldn't do it unless the photograph is released for that use or you have the photographers permission. There are probably other sites which state other points of view, but I didn't see any in a quick scan.
There probably is some case law somewhere, not sure how important it is. If you get asked decide where you stand then politely say yes or no. If you don't get asked and you actually find out about it ... guess that's a tougher question that falls in the category of "crossing the bridge when you come to it".
Then there is always another point of few ... even if it is legal it doesn't seem right to use a photograph without permission.