We have to be careful here in answering your question because there are actually TWO movies that correspond to the short story "The Secret Life of Walter Mitty." Because one of those movies was created in 1947 and the other was created just two years ago (in 2013), I am going to assume that it is the more modern of the two films that you're interested in. In that case, you are right, both people and landscapes are focused upon in the film. Let's look at a few examples that you could use for support in the context of an essay.
First, let's look at character by using the new love interest of Cheryl (who is NOT the wife in the book, by the way) as an example. Mitty works with Cheryl, and he has a crush on her. There are numerous situations where Mitty's daydreams run away with him and he suspects that Cheryl is secretly seeing her ex-husband. However, by the end of the film, we learn that Mitty was so involved in his daydreams and lacked a sense of reality to such an extent that he was in error. The ex-husband is simply a refrigerator repairman and comes to fix the appliance. As a result of this knowledge (and their bonding over the news of "negative 25" in Life magazine), the two become a couple. The technique of the close up reinforces this, as we zoom in on them holding hands at the end. However, it is ambiguous in the sense that the close up itself could just be another one of Mitty's daydreams.
Next, let's look at landscape. The perfect example of landscape in reality is the movie poster: a scene where Walter Mitty is quickly using the crosswalk at Time Square in what seems to be New York City. It is here that the movie's theme comes across well:
Stop Dreaming, Start Living.
This is the reality of Walter Mitty's life and contrasts greatly with the context of his adventures and daydreams, for those landscapes are Greenland and Los Angeles and even the shark-infested waters of the ocean! The implied idea behind these landscapes is that Mitty (similar to the book) lives within his fantasies instead of in reality. By the end we are led to believe that Mitty will find some adventures in reality as well.
In conclusion, it's important to note that this romantic comedy is only loosely connected with the original short story. Yes, the focus upon daydreaming is the same, but due to the advent of new technology focused upon in the film, the stories cannot be identical. This is reflected in both people and landscapes.
The film Walter Mitty is not the first film to use nature as an avenue for escapism. All kinds of movies and literature have used that plot motif for hundreds of years. In this regard, the film is not unique. In my popular culture studies class, we call it the "Myth of Rural Simplicity." I've included a link below to a popular culture media text that has a section on this myth. It starts on page 123 of the text.
In summary, the myth says that life in the country (out in nature) is simpler, easier, and more peaceful than urban life. There are all kinds of real life examples of how this myth is perpetuated. It's how Thomas Kinkade has made his entire artistic career. Most of his pieces are of some rural natural scene, and the viewer can't help but feel peaceful while looking at his art. Other examples would be annual camping trips that families make, or trips to National Parks like Yosemite. The draw is that a person can find inner peace by communing alone with nature. He/she is not bothered by all of the noise, people, and rushing that city life has. Your question stated it perfectly. Nature offers people an escape from their busy lives. It's not a pill to be taken, but simply a comfort to be received.
I stated earlier that other films and books have made use of the rural simplicity motif. The "Twilight" series (book/movie) is a good modern day example. It seems that any time that Bella and Edward head into town together something bad happens, but alone in the woods is where the two of them find peace and love with each other. Orwell used it in 1984. Julia and Winston sought out a place in the woods to free themselves from the oppression of Big Brother. It's possible to apply this myth to the Bible as well. Jesus Christ spent 40 days alone in the desert. Entire TV shows have been built on the myth. Shows like "Lassie" and "Little House on the Prairie" are all about rural simplicity. Hunger Games protagonist Katniss Everdeen frequently will disappear to hunt with Gale. Sure, they need food, but it's also the only time in the novels where she feels calm and free. Disney is not immune to the myth either. In "Frozen" Elsa doesn't run away to another kingdom. She runs away to nature in order to be herself and find peace. Nature allows her to let it go (pun intended).
There are deep historical roots to this myth too. Probably the largest influence came during the Romantic movement of the 1800s. Authors like Wordsworth, Longfellow, Emerson, and Thoreau all loved nature. To them communing with nature could give them inner peace and knowledge. In a way, it offered Romantics and Transcendentalists a type of religious experience to be "one with nature."
The picture that you attached doesn't quite support the Romantic ideal of communing with nature, but it's close. The reason it isn't exact is because the photo focuses on the human. He is central to the picture. Romantic art of the 1800s can have people in it, but they are often small and not centered. I've attached a link to an example. The photograph does fit with modern day rural simplicity. It shows a man, alone, in a natural scene. While he doesn't look happy, he also doesn't look sad or angry. He seems to be in contemplative contentment. He is immersed within the hold of nature and has found peace. He is able to focus himself on what is important without being bothered by hustle and bustle of the outside, city world. He has found simplicity in a rural setting.