I believe so! Lemmy will try to generate a thumbnail from the link field whenever possible. You can try this by creating a post with a link and see what shows up after you make that post.

If you don't see a thumbnail after creating the post, then it's likely that Lemmy wasn't able to automatically fetch the thumbnail for it. If this happens, you can specify a custom thumbnail for Lemmy to use using the Thumbnail URL field (as long as this points to a proper image)

The posts have images in their body of the text too.

You can have images within the body of the text, but Lemmy won't use those images for the thumbnail (if I recall correctly) - it'll only look at the URL field to determine what thumbnail to use.