It was taken at the 5th annual Solvay Conference.
From the wiki page:
Seventeen of the twenty-nine attendees were or became Nobel Prize winners, including Marie Curie, who alone among them, had won Nobel Prizes in two separate scientific disciplines.
Here's the full page on the conference, including a list of names for those in the picture you don't recognize:
http://en.wikipedia.org/wiki/Solvay_Conference
Just thought it was neat and worth sharing in case anyone hadn't seen it before.