Describe the poet's use of postcards as the central image of the poem "Postcards from Kashmir."

The poem uses the image of a postcard as a metaphor for fond and idealistic memories. The poem begins with the speaker receiving a postcard from Kashmir, a geographical region in northern India. The speaker is from Kashmir, so he states, "my home a neat four by six inches," which is the size of a postcard. Without telling us exactly what the picture on the postcard shows, the poet says, "Now I hold the half-inch Himalayas in my hand," so we at least know that it pictures a mountain range. The poet describes this as an example of "neatness," which he views favorably. Yet this seems like an unemotional understatement. We feel a poignancy in the thought that "home" can be reduced to a "neat" two-dimensional format small enough to fit in a mailbox. Surely the poet considers "home" to be much greater than that.
The poet clarifies this point in the third stanza, but ironically, he now presents the postcard as an exaggeration of home. The speaker knows that when he has a chance to return to Kashmir, the water won't be as deep a blue as on the card and the other colors of the landscape won't be as brilliant. But by stating "my love so overexposed," we understand that the speaker has idealized memories of his homeland that he knows are better than the realities that exist there.
Thus the image of the postcard is both an understatement and an exaggeration of "home" to the speaker. The card can't possibly contain all the strong feelings and memories he has, but it also depicts a representation of home that is more beautiful than his actual experiences were. The speaker obviously loves his home and cherishes its beauty while acknowledging that his memories are enhanced by homesickness.
https://literarydevices.net/understatement/

https://www.britannica.com/place/Kashmir-region-Indian-subcontinent