it’s not. the lemmy-ui max width for the poster element is far smaller than that (1104x960). in fact, the poster element is set to be a near-square (displays as rectangular in web and mobile web on the page header), as it also displays in the sidebar and in mobile apps as a square if the image is. most mods simply assume it’s a rectangle and upload a rectangular image.
this image is made to be the largest usable resolution lemmy can display as a community poster and optimized to be very small in file size. see on lemm.ee/c/plex