Who it is
The mystery woman in the middle of yesterday's picture? Kes from Voyager (played by Jennifer Lien). The guy who was in yesterday's picture with the Cornell shirt is Neelix (played by Ethan Phillips), and the other guy is, of course, Robert Picardo.
Even if you don't know who Kes is, you've gotta admit that she looks very different if you compare this picture to yesterday's picture. When I first saw it, I thought she was a fan--not part of the cast. So you can understand how incredulous I was to find out who she was.