It was created for publicity purposes-distribution to the media. The image was meant to bring attention and publicity the singer, the same as the publicity photos for actors and actresses in the film industry were intended to do.
Film production expert Eve Light Honthaner in The Complete Film Production Handbook, (Focal Press, 2001 p. 211.):
"Publicity photos (star headshots) have traditionally not been copyrighted. Since they are disseminated to the public, they are generally considered public domain, and therefore clearance by the studio that produced them is not necessary."
"There is a vast body of photographs, including but not limited to publicity stills, that have no notice as to who may have created them." (The Professional Photographer's Legal Handbook By Nancy E. Wolff, Allworth Communications, 2007, p. 55.)
"Publicity Photos (star headshots) older publicity stills have usually not been copyrighted and since they have been disseminated to the public, they are generally considered public domain and therefore there is no necessity to clear them with the studio that produced them (if you can even determine who did)."
United States Copyright Office page 2 "Visually Perceptible Copies The notice for visually perceptible copies should contain all three elements described below. They should appear together or in close proximity on the copies.
2 The year of first publication. If the work is a derivative work or a compilation incorporating previously published material, the year date of first publication of the derivative work or compilation is sufficient. Examples of derivative works are translations or dramatizations; an example of a compilation is an anthology. The year may be omitted when a pictorial, graphic, or sculptural work, with accompanying textual matter, if any, is reproduced in or on greeting cards, postcards, stationery, jewelry, dolls, toys, or useful articles.
There is controversy about its date. Commonly said to be a 1928 instrument, but researcher Joe Spann concluded that it couldn't have left the factory earlier than April 1929 and was shipped January, 1930.
References
InfoField
↑Alcantara, Paul. 85558- Gibson L-5. prewargibsonl-5.com. Archived from the original on 2017-05-04. Retrieved on 20 May 2018. "Joe Spann...Spann's Guide to Gibson...I conclude that Maybelle's guitar was shipped in January of 1930."
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.