I seem to have increasing instances of images from website pages linked in Facebook posts not whosing up. I go to the Facebook debug tool: https://developers.facebook.com/tools/debug/sharing, paste the page I've shared and see the link as I would expect it to appear on the Facebook post, but it doesn't - the photo is missing.  There is a warning given on the Facebook debug page "The 'og:image' property should be explicitly provided, even if a value can be inferred from other tags.".    This is only something that has started happening in the past few months. Anyone any clues? Eileen