Apparently, an image whose part above the fold is indeed the largest, but of which only a small part can be seen, can be considered an LCP element, even if another image has a larger part visible... 😭
You can see this for my photography site in this WebPageTest result:
https://www.webpagetest.org/vitals.php?test=230606_BiDcN4_4NJ&run=3&cached=0#lcp