Sorry to butt in, Peter, but alt text is not a featured article requirement. See Wikipedia:Featured article criteria, (number 3. Media). Alt text was a FA reqirement, briefly, about three years ago, and this caused so many difficulties, disagreements and squabbles that it was withdrawn as a criterion and became voluntary. You have discretion here, but for the sake of consisrency it should be all or nothing; if editors choose to use alt text, then it has to be used in all that article's images, not just some.