Post icon images: URL vs image

We see posts everyday, some (most) with a square-cropped image to the left. Some of these post images, let's call them "post icons", are there because on the "Create Post" form there is a text entry field explicitly pointing to the author's uploaded image. Otherwise, the post icon is an image from the link entered in the URL text entry field.

Are you with me so far? I hope so.

Now, sometimes the URL returns an image and sometimes it doesn't. If it doesn't, a default Lemmy link icon appears.

Here's an example. The linked page has an image but it doesn't appear as a post icon.

1. How can I get the URL to return an image I know is on the linked page. Not necessarily a specific image but any image from the link.
2. Is there a way to use an author-chosen image when no linked image exists (or if I'd like a different image)? I've tried using both the URL and the Image fields but unfortunately the Image link takes preference.

I hope I've been clear. Please don't hesitate to ask questions. Thanks!

EDIT: added the post icon image.