This is the cover art from the April 1934 issue of Black Book Detective. The artist is Rafael de Soto, a prolific pulp illustrator and the purported descendant of infamous conquistador Hernando de Soto. (From that same link, de Soto’s tube-girl-to-mermaid mad science transformation cover for the May 1940 Terror Tales magazine is also not to be missed.)
Image provenance by Bacchus at Erosblog.