Camel Postman is the correct term. Changing it to Dromedary and Rider would ignore over a 125 year tradition of calling it Camel Postman and the fact that it indeed does depict a postman. Besides dromedary is a type of camel.
http://www.pjsymes.com.au/articles/CamelPostman.htm
An excerpt:
In 1897 On meeting Captain Stanton, Kitchener ordered him to provide a design for a postage stamp and promptly gave Stanton a generous five days to complete the design. Inspiration eluded Stanton for several days, until the regiment’s mail was delivered by camel instead of the usual river steamer. As well as being a map-maker, Stanton was also the liaison officer for the local tribesmen, which included the Howawir tribe. Therefore, he was able to prevail on the Sheikh of the Howawir tribe to don his war kit and mount his camel, so that he could draw his inspired image of the ‘Camel Postman’. In order to make the camel appear to be on a postal run, bags filled with chopped straw were attached to the saddle to imitate mailbags. Stanton then drew the sketch of the Sheikh riding through the desert and added the names of ‘Khartoum’ and ‘Berber’, two towns in Sudan, to the mailbags.
