Hi,
The LCP image on the site is made to be a background image by the theme, and not an image element. That may be the reason why you’re struggling to find it.
The image is called from a CSS file and not from the homepage (likely called theme.liquid) of the site.
Because the LCP is a background image, there’s not much you can do from a loading attribute standpoint. You could try to preload the image, but that usually doesn’t help a whole lot in terms of the load speed score; especially on the mobile side.
It might be worth it to change the image to an image element. I think that would help the speed score and allow you to add a loading attribute of eager to it.