If you include a link within a Mastodon post, it will automatically show a "link preview" below the post.
Link previews often show an image associated with the link and/or basic information about the contents of the link.
You don't have to do anything special, the preview is generated automatically. It may sometimes take a few minutes to appear if your server is busy.
More info about previews as well as many questions answered at:
➡️ https://fedi.tips/how-do-i-get-a-link-preview-to-appear-in-my-post-on-mastodon
