<p>Photographic portrait of Major-General <a href="https://en.wikipedia.org/wiki/Hugh_Trenchard,_1st_Viscount_Trenchard" class="extiw" title="en:Hugh Trenchard, 1st Viscount Trenchard">Hugh Trenchard</a> in the uniform of the Royal Flying Corps
</p>
The photograph was scanned from Probert, H. (1991). <i>High Commanders of the Royal Air Force</i>. HMSO. <a href="//commons.wikimedia.org/wiki/Special:BookSources/0-11-772635-4" title="Special:BookSources/0-11-772635-4">ISBN 0-11-772635-4</a>.  The entire book, including its photographs, was issued under Crown Copyright.  Although copyright for the book is dated at 1991, the Crown Copyright on the photographs starts from the date when they were taken.  As this photograph is more than 50 years old it is now in the public domain.
24 March 1916 (promoted (temp.) Maj Gen to 1 April 1918 (in RAF, not RFC). Given as 1917 by <a rel="nofollow" class="external free" href="http://www.militaryimages.net/photopost/showphoto.php/photo/6153">http://www.militaryimages.net/photopost/showphoto.php/photo/6153</a>
Probert, H. (1991). <i>High Commanders of the Royal Air Force</i>. HMSO. <a href="//commons.wikimedia.org/wiki/Special:BookSources/0-11-772635-4" title="Special:BookSources/0-11-772635-4">ISBN 0-11-772635-4</a>.