The image projected at the end was not Gloria Swanson—it was Elaine Paige! Each actress who played the role, including the covers, shot footage used for that closing moment.
I love that you described it as a “young” Gloria Swanson though. I always saw the moment as contrasting young Norma Desmond with the mad woman we saw before us as the metaphoric curtain closed. Others, though, have insisted to me that it is not supposed to be footage of young Norma Desmond, but the close up for which she tells Mr. DeMille she is ready.