The photo is in the public domain in the U.S. and does not need a free use rationale, but here is one for doubters. {{Non-free use rationale| Article = Nazi human experimentation | Description = "Forced cold water immersion experiment claimed to be at Dachau concentration camp presided over by Professor Holzlohner (left) and Dr Rascher (right). The subject (a prisoner who was forced to participate) is wearing a Luftwaffe garment. Note the floating blocks of ice." | Source = Taken from: Hanauske-Abel, Hartmut M. "Not a Slippery Slope or Sudden Subversion: German Medicine and National Socialism in 1933." BMJ: British Medical Journal 313(7070): 1453-1463. 7 December 1996. The journal does not claim ownership of the copyright. | Portion = All | Low_resolution = Yes | Purpose = To illustrate practices of human experimentation. | Replaceability = Non-reproducible historic image with no free replacement, illustrating the subject of prisoner abuse with cold water immersion in the Holocaust article. The German government commissioned the picture and owned the copyright.

All Nazi era copyrights were seized by the Allies and are public domain in the US. according to the US National ARCHIVES. see US government statement on captured German records

Its use on Wikipedia is no doubt "fair use" for its historical purposes in illustrating war crimes.