Is there a word to describe spatial/height imagery?

You're talking about conceptual metaphors.

Conceptual metaphors are part of the common language and conceptual precepts shared by members of a culture. These metaphors are systematic because there is a defined correlation between the structure of the source domain and the structure of the target domain. We generally recognize these things in terms of a common understanding. For example, in our culture, if the source concept is "death," the common target destination is "leave-taking or departure."

[Nordquist; ThoughtCo]

Here the source concept is socioeconomic class, the target destination altitude, and you are using an orientational metaphor.

From the same source:

Cognitive linguists George Lakoff and Mark Johnson have identified three overlapping categories of conceptual metaphors:

• An orientational metaphor is a metaphor that involves spatial relationships, such as up/down, in/out, on/off, or front/back....

I think the word that might be closest is "stature," but with a tiny bit of tweaking for usage.

The use of purposeful stature in this scene, illustrated through the protagonist 'kneeling and bending' while Alex is on his tiptoes, highlights the socioeconomic divide between Alex and the protagonist.

VS

The purposeful contrast in stature in this scene, illustrated through the protagonist 'kneeling and bending' while Alex is on his tiptoes, highlights the socioeconomic divide between Alex and the protagonist.