No permission is required.

1. First, the photo is a mechanical scan/photocopy of the original cover and does not qualify for independent copyright protection.
2. Second, the portrait was first published prior to 1978 without a valid copyright notice. Opus Posthumous was first published in 1957; the hardcover book itself carried a copyright notice, so its contents remain copyrighted. However, the first-edition dust jacket did not carry a separate copyright notice. According to The Compendium of U.S. Copyright Office Practices: Chapter 2200, § 2207.1(C) at p. 15:

"A notice of copyright on the dust jacket of a book is not an acceptable notice for the book, because the dust jacket is not permanently attached to the book. Likewise, a notice appearing in a book is not an acceptable notice for the dust jacket or any material appearing on that dust jacket, even if the book refers to the jacket or material appearing on the jacket."

Keep in mind that the pre-1989 requirements for copyright notice were highly formalistic and, other than a few enumerated exceptions, required these three elements:

1. "The symbol © or the word 'Copyright' or the abbreviation 'Copr.' or an acceptable variant such as "(c)";
2. "The year of first publication for the work"; and
3. "The name of the copyright owner, or an abbreviation by which the name can be recognized, or a generally known alternative designation of the owner."

If just one of these elements is omitted, the work is deemed to have been published without notice and is not eligible for copyright protection. None of these elements—the name "Sylvia Salmi", the year of publication, a copyright symbol (or any acceptable variant)—appear anywhere on the dust jacket. The identifications of the publisher and author do not meet these requirements. The full dust jacket can be seen at the Internet Archive (registration required to see the full ebook, and there may be a waitlist for free access, but the front cover, front flap, and back cover can be seen in preview).

A cropped portion of Salmi's photograph was used for the cover of Collected Poems. This image has been cropped from a high-resolution scan of the original uncropped photo so that it matches the dimensions of the Collected Poems cover. The most conspicuous difference is that the original uncropped portrait shows more of Stevens's crossed arms. It is unclear whether the "uncropped" portions from the original photo are under copyright protection or not; without further information they should be presumed to be copyrighted.