Well, they are all Europeans. I am not denying that racism existed at the time, but hardly any non-white person lived in Europe back then. (Even nowadays in most European countries the percentage of non-white people is so low that, in a group of 30, finding none is well within statistical error).

Interestingly, many (if not most) of people on that conference were Jews, which were often treated as second-class citizens even before Hitler.

Also, an additional testament how Marie Curie was exceptional was that she was an immigrant from Poland (which didn't even exist when she was born; her initial studies were in secret), and that she is still the only person ever to win two Nobel prizes for two separate fields (physics and chemistry).

If you want to go down that route - which is definitely your right - you could re-shoot a similar picture today and it probably would not show a major shift in demographics. Sad but true.