There’s only one URL field for posts, and it can either be to an image or a website.
Posts with images and links are usually to news or other websites that provide a thumbnail image. Lemmy backend will pull the link’s metadata and save the returned thumbnail image if it has one. That’s how article links with the headline image show both an image and url.
TL;DR: The thumbnail image for link posts is generated by Lemmy itself and you can’t provide both.