I seem to remember that Linda McFly's sweatshirt (in the vacation photo) sometimes says 1984 and sometimes another year (1991?). I don't have a screencap and I can't find anything about it online. Is it like that or was I seeing things?
Futurepedia says that she did graduate from Hill Valley High School in 1984.
Regarding the actual photograph, see it here with Craig vanished. It's a little bit clearer and looks a little more like '84'
Bear in mind that it can't be '91', because quite simply that Marty came from 1985 - how could he have a photograph of his sister wearing a sweatshirt with 'Class of 91' if he was only from 1985!