The front cover includes Stoltenberg, Amin, Japanese Emperor Akihito, and a famous Aboriginal-Australian politician, a combo of political leaders put together with the purported aim of "illustrating different races", the NTB news agency reported on Friday.
Stoltenberg, whose Labour Party survived the right-wing terror attacks of Anders Behring Breivik last year, was not aware that the picture had been downloaded by the British publishing company.
The publisher Ostara Publications claimed there were no usage restrictions on the image, a statement that a Norwegian copyright expert contested on Friday.
"The image is for use in contexts that concern Stoltenberg," Norwegian media expert Jon Wessel-Aas told NTB. "If it had been a book about prime ministers or about Jens Stoltenberg it would have been different."
The website of the Norwegian government has seven official portraits of Stoltenberg for editorial use, with a link to his private Flikr account.