The photograph is a well known one of King Chulalongkorn cooking on the rear porch of his Ruen Ton residence at Dusit;
Interesting read about the story behind this photo and the Royal Siamese Consorts , with cameras:
"In this photo, the king’s attire greatly contrasts with the uniforms typically worn in his official portraits. Here, he wears no shirt or jacket, but only a loosely wrapped lower garment called a pha khao ma—an outfit which could be worn by any Siamese commoner. His pose is casual and unguarded; the king is shown sitting on a chair as he bends intently over a wok, stirring it with his right hand while he smokes a cigar or cheroot with his left. The overall scene has a relaxed and contemplative feeling, as if one is present with the king in an intimate domestic moment. It is unsurprising that this photo is frequently seen today in restaurants throughout Thailand, in the area of the shop reserved for venerating an image of the king, past or present.[26] The intimacy of the image affords the viewer a vicarious closeness to a much-revered monarch, while simultaneously rendering him all the more human and accessible via the quotidian quality of his dress and activity"