Quick tech tip for WordPress bloggers: If you’re sharing a post on Facebook and you can’t select a particular image and display it as the thumbnail next to your link, the problem is probably that the image is too small. Images have to be at least 200×200 pixels or Facebook will choose another image (if there’s another available to choose).
Solution: make sure the image in the media library is at least 200×200. You can resize it in your post to be smaller if you want – that’s OK. You may need to refresh Facebook’s cache to make the new image appear. You can do that by loading your post in the Facebook Debugger.
This tip applies to all web pages – Facebook expects a thumbnail image to be at least 200×200, even though it’s much smaller when displayed. Whatever.