The picture shows the serious lesions of burns of the genitals and thighs of a soldier of the French 22nd Infantry Regiment. On 18th March 1918 he got the full blast of a German gas attack. The musterd gas burned right through his clothes.
This French autochrome color photograph was made in 1918 and kept for medical purposes.
Very few pictures depict the real horror of poison gas. When the people at home would have known what gas really did to soldiers, they probably would have tried to stop the use of it — and maybe the war altogether. That's why pictures like these were never published or shown in public.