This is an original cover illustration with artist uncredited, but by (I believe) based on the style, Gene Szafran.  This artist's published work is as distinctive in style as it rarely available.   The book "The World's Strangest Mysteries" was edited by Rupert Furneaux, and published by Ace Books in paperback, 1975  (see cover for reference)  The painting is unsigned and is in mixed media, watercolors, pastels, inks; the image 19.5" x 11" on illustration board 24" x 14.5" with 3 small spots of black ink top of one face and in the background (and unknown whether the artist was responsible or not)  The title of the book has been written bottom right below the image, in pencil.  This is a very handsome painting, very representative of Szafran's style