How does Facebook detect images when adding a link?

Question

When you add a link to your Facebook page, after some processing, Facebook presents you a next/prev button to choose an image linked to the url your are inserting.

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Answer 1

I am not familiar with the way Facebook did it, but I can tell you how I would do it.

1. Establish web request to desired url
2. Parse resulted web response with regular expression, that looks for <img src="" />

Answer 2

This looks like a duplicate of: Facebook Post Link Image

Use <link rel="image_src" href="abs_url_of_your_image">

Answer 3

My guess, is that FB hides repeated images - 'couse a repeated image normally indicates a design-element (not content). When you post a link, only images related to that articles content, are relevant.

So; make sure your image only appears once, and see if it helps!