Images for 'open graph' tags
Twitter seems to need an 800 by 800 pixel image for the image in its 'meta' open graph tags. Twitter says just to create a post and put in a link to see how the 'card' for the meta tags look. Sometimes the 'card' is shown but sometimes it is not. Twitter has an old tool to show which 'meta' tags are being used, but Twitter now says this is not always correct on all devices. At first I scaled down some old 1280 x 720 pixel screenshots to 800 x 800. But the text looks a bit tall and narrow, so I will have to take fresh screenshots that are square. Facebook seems to look for an image in the page referenced in the 'og:url' tag rather than the current page. Facebook has a tool to check which 'open graph' tags are used. I have added 'open graph' meta tags to all my pages.