Pictorial qualities, in regard to literature, is otherwise known as imagery. Imagery is the use of the author's language meant to allow a reader to create a mental image of what is being described. Many times, an author's text appeals to a reader's senses: sight, sound, taste, touch, and smell.
In regards to Samuel Taylor Coleridge's "Kubla Kahn," the poem is filed with language which appeals to a reader's senses (which, in turn, allows readers to create a mental picture of what is being described).
The very opening of the poem sets a distinct picture. The poem is a "vision in a dream." Engaged readers may initially picture an image blurred at the corners and sides (similar to the way movies depict dream scenes).
Where Alph the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
So twice five miles of fertile ground
With walls and towers were girdled round:
And there were gardens bright with sinuous rills,
Where blossomed many an incense-bearing tree;
And here were forests ancient as the hills,
Enfolding sunny spots of greenery.
This stanza allows readers to picture the Alp, running through shaded caverns. Rich fields and fertile gardens follow the river, which wraps like a snake through the fields and into the forest.
Given the descriptive words Coleridge chose, a reader can see exactly what is being described. These exist as the pictorial qualities, or imagery, of the poem. Readers can see that this place is one much suited for the construction of the "pleasure-dome."