Lemmy offers image thumbnails for some but not all. We have something in the works right now to remedy this.

Hello! Web developer here.

I asked myself the same question.

Usually, when you share a link on social networks, the app/website read some metadata in the head of the page to build a card that shows title, description and image. It’s not mandatory for website to work so not everyone does the work even though it can be easily automated.

Now there are different way to add data and maybe Lemmy don’t look for every possibility here.

One quick verification you can do is trying to share the link from a browser to a social network and see if it produces a card with an image or not.