Just to add, there seems to be some issue with thumbnails on .cafe, from the admin:

Yes, there’s clearly some problem with the change introduced in lemmy 0.19.7 where thumbnail generation was changed to force a smaller size (and potentially something else). Still not entirely sure which part - lemmy or pict-rs - is misbehaving, but working on both fronts trying to find the culprit. Nothing so far, unfortunately

So currently, most new posts don’t have an icon.