My experience with Facebook made me doubt that computers are deterministic machines :)
Sometimes it will accept image of any size (between 200 and 1500px), sometimes complain that image is too small (og:image should be larger) even if image is squared and bigger than minimum size (e.g. 400x400px).
I had situations where the same link showed all 3 og:images in linter and in FB status field, then suddenly 2 weeks later stopped working and started showing only one image while trying to share it in FB status field (linter still showed all 3 images), and vice versa.
Sometimes FB seems to have problems with different domains (e.g. page is on www.example.com and og:image on www.picdumpexamplesite.com), sometimes it doesn't mind...
IMHO it's heavily broken, and the best you can do is:
- make images squared, rounded to "hundreds", e.g. size 300x300px
- host them on the same server (same domain name)
- use jpg
- avoid any "strange" chars in filename (I use only letters and numbers)
- pray that something of the above works :)
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