This file is in the public domain, because the image, owned by the Institut International de Physique
Solvay, was published in Belgium more than 70 years ago.
In case this is not legally possible:
The right to use this work is granted to anyone for any purpose, without any conditions, unless such > conditions are required by law.
Wikipedia states that:
So it should be no problem. More details here: http://en.wikipedia.org/wiki/File:Solvay_conference_1927.jpg