This is an excellent poem in which the speaker explores the way that even the most familiar of surroundings can become like "desert places" where he is overwhelmed with loneliness and questions his own significance as a human being.
When analysing imagery, it is important to remember that imagery is defined as pictures that are painted with words that create an image in our mind of what the author is describing. Often imagery will appeal to the five senses to make that picture more vibrant.
Thus it is that this poem, which is, after all, about emotional desolation, creates powerful images of the snow and the way that it erases characteristics and creates a bleak landscape. Notice how the speaker describes what he sees:
A blanker whiteness of benighted snow
With no expression, nothing to express.
Snow is personified as a human face, but a face which has "no expression" and "nothing to express." The whiteness of the snow is "blank" and is shown to erase or cover up or "smother" all familiar features. This creates an intense feeling of "loneliness," which is reinforced through the repetition of this word in stanzas two and three. All of these examples combine to create a very bleak picture indeed of emotional desolation and the desert places that surround us even when we know where we are.