Note from 6 June 2023

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

Screenshot from WebPageTest showing the image mostly hidden behind another one is the LCP