You know that little information thing below posts telling you which other communities a specific post has been posted to?

That one, won't appear if you crosspost a pure-text post. Sure you'll get a link in the body, but lemmy won't "intuitively" know the two posts are connected.

By using an image in the URL of the post, not only do you give your text post a thumbnail, but you allow lemmy to easier link them together when cross-posting.

If you're wondering "But where the hell do I find an image for each post", well, Generative AI to the rescue! I always just fire up Lucid Creations and use it to generate something fitting to the subject, or just something pretty at 768x512.