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I am curious how Facebook selects a substitute image, when the one specified in <meta property="og:image" /> is considered invalid according to the linter. It is said that if og:image is missing or invalid, Facebook will acquire any other images on the page that meet size/format/resolution criteria the best. However in my cause, such an image was chosen that was not referenced anywhere on the pages, although it was present on the server and publicly accessible — so that is probably not the case. Does it maintain some cache of once published pictures, or crawls other pages, or what?

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You'd probably need a Facebook engineer to answer that. We're not privy to Facebook's inner workings. –  Al E. Jun 14 '13 at 13:23

1 Answer 1

Source: http://ogp.me/

og:image:url - Identical to og:image.
og:image:secure_url - An alternate url to use if the webpage requires HTTPS.
og:image:type - A MIME type for this image.
og:image:width - The number of pixels wide.
og:image:height - The number of pixels high.

Example:

<meta property="og:image" content="http://example.com/ogp.jpg" />
<meta property="og:image:secure_url" content="https://secure.example.com/ogp.jpg" />
<meta property="og:image:type" content="image/jpeg" />
<meta property="og:image:width" content="400" />
<meta property="og:image:height" content="300" />
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I know that. I'm asking how does Facebook choose alternative images if the specified one does not meet criteria / declined by lint tool. –  Actine May 15 '13 at 20:47
    
The documentation doesnt make it clear if we need to specify the width/height of the source image or not or, and pertinen to YOUR question, does the setting of these for other images make them MORE likely for auto selection when your choice is deemed inappropriate. Like seemingly hundreds of others I have an issue where the FB Debugger is not complaining about my og:image but it's selecting an older "safe image" for some unknown reason. –  Richard Riley May 29 '13 at 1:20

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