I had to look up ekphrasis; somehow it was not covered in my literary education. I found this description on a page of Oregon State University: “Basically, an ekphrasis is a literary description of art.”
The key idea there is literary. The art work exists, or is presumed to exist, outside the particular work of literature, but the description of it serves some literary purpose.
The Picture of Dorian Gray came immediately to mind, and I looked far enough to see that this fictitious portait is indeed considered an ideal instance of ekphrasis; so the described work of art does not have to exist in the real world. Here, Oscar Wilde uses the painting to mirror the moral dissolution of his character while the living Dorian Gray remains astonishingly young and beautiful.
Another familiar example is John Keats’s “Ode on a Grecian Urn,” in which the poet describes the figures painted on a Grecian urn as being captured in their prime and preserved forever by the artist’s depiction of their beauty, while we in the living world will fade, grow old, and die.
Does this help? I can dig deeper if necessary.