Imagery is defined as descriptive words that develop either concrete, sensory (feel, taste, see, hear, smell) mental images or mental images of abstract qualities (goodness, admiration, etc). Allegory is defined as a piece of writing in which characters and actions have universal symbolic meaning and may be interpretively isolated within non-allegorical texts (allegoresis).
An obvious aspect of Hemingway's story that yields to allegorical interpretation is the absence of names for the characters. Though not capitalized as is "Everyman" in Everyman, the waiters and the deaf old man are only ever identified by universal titles: "the old man," "one waiter" aka "the unhurried waiter, the older waiter," and "the younger waiter" aka "the waiter who was in a hurry, the waiter with the wife." This endows them with an allegorical, universal presence. In essence, then, they stand in for three types of men in the post-world war world as Hemingway saw it.
The most pronounced imagery occurs at the beginning of the story and sets the scene for the waiters' existential discussion of the value of life: for the deaf old man, it is brandy and a pleasant place to forget; for the younger waiter, a night at home with his wife; for the older waiter, giving comfort to someone else ("I am reluctant to close up because there may be some one who needs the cafe.") and himself.
The opening imagery, that is expanded now and then throughout, is concrete and supports their existential journey.
It was very late and everyone had left the cafe except an old man who sat in the shadow the leaves of the tree made against the electric light. In the day time the street was dusty but at night the dew settled the dust, and the old man liked to sit late because he was deaf and now at night it was quiet and he felt the difference.
The concrete imagery (based upon concrete physical objects, not abstractions) in this opening passage establishes a late, dark night; a tree; shadow of leaves; an electric light; fallen dew over the dust of day; quiet.
a table that was close against the wall near the door of the cafe and looked at the terrace where the tables were all empty except where the old man sat in the shadow of the leaves of the tree that moved slightly in the wind. A girl and a soldier went by in the street. The street light shone on the brass number on his collar.
A bit later, to the image is added a table; a wall; a door; a terrace; empty tables on the empty terrace; again, shadow of leaves; a light wind; a girl and a soldier passing by; a street light; a brass number on the soldier's collar.
This is a clean and pleasant cafe. It is well lighted. The light is very good and also, now, there are shadows of the leaves.
Finally, to the image are added cleanliness; pleasantness (abstract quality); lighting; goodness (abstract) of light; and again, shadows of leaves.
The totality of the imagery is an image that is bleak, with one electric light and one tree; lonely, with one girl and one soldier nearby; quieted by night dew; with the appearance of goodness and cleanliness; dark save for the cafe. The one point of interest is the "shadow of leaves" that enters every description. This bit of imagery, then, becomes a symbol of living life in a shadow, an appearance, of substance without real substance. This growing image of desolation and aloneness, doubling as a symbol, then, supports the theme of "nada y pues nada," nothing and more nothing.