I share it on another site that generates a thumbnail and then copy the thumbnail to Lemmy.

There’s a slightly different variant I’ve seen someone use, which I think they get by hovering over the share button. But I’m not sure how they copy it, because it seems to have disabled the “copy image” option. Maybe they use their browser’s Inspector feature.

edit: actually, in retrospect, I don’t think that is the source of the other person’s image. But it’s been a while since I’ve seen their variant, so I dunno where they get it from.