I could find next to nothing about Donald Masefield Easton (1896-1956) on the first few pages of a Google search, and almost no examples of his work.
The only reason I made the search and decided to write this post was because I finally discovered who illustrated one of my favorite vintage Hawaii tourist posters, the one you see above (slightly cropped from a photo I took). One of these posters was displayed in Honolulu's Royal Hawaiian Hotel along with a small plaque noting the artist's name. I had seen the poster elsewhere over the years, but without any indication of who did it.
As for the poster itself, I like it because of its early 1930s view of Waikiki when the only major buildings were the Royal Hawaiian (left) and Moana (right) hotels, and because it features a whiff of those toned-down color schemes popular during the 1920s. Clearly a case of false-nostalgia on my part.
The examples shown here really aren't sufficient for a serious evaluation of Easton. Because they deal with the west (Union Oil was a West Coast firm), I'll assume for now that he was based in California, was able to make a living as a competent illustrator, but never made it to the New York - Chicago "big time" for illustrators of his generation.