Also review again where Open Graph images come for forum topics, news articles, and catalogue entries. Make sure it works well.

Hi Chris. This was the link I couldn't find earlier when I mentioned Twitter Cards @ https://developers.google.com/web/fundamentals/discovery/social-discovery/

Same as Facebook Open Graph except for:-
<meta name="twitter:card" content="image">
<meta name="twitter:site" content="@agektmr">

Potentially also:-
<meta name="twitter:creator" content="@yuwskei" />

It does require domain approval @ https://cards-dev.twitter.com/validator

Right. I think that's something I can document in a tutorial, but adding that to core would be bloat. It's very specific.

I've not quite done what it says in the description - instead I've made things much more powerful and configurable, and better documented. This way people can set all kinds of wonderful schemes they wish for via template editing.

This was hard, a lot of changes!

1) Added support for making picture fields take priority, via a new field option
2) Changed hard-coded <img> tag scanning for forum posts to work via a symbol (METADATA_IMAGE_EXTRACT), and deployed this in a number of content templates - meaning it is now more configurable
3) Smarter filtering out of invalid metadata, so valid fallbacks can take precedence; including applied file extension check for specified Open Graph images
4) Added minimum image size check for Open Graph images also
5) Made it easier to override images used for different content types in HTML_HEAD.tpl
6) Reworked documentation, merged explanations from 2 different tutorials together, and rewritten to be much clearer
7) Written better explanations for how to override for different content types, and how to just have a flat choice of the Open Graph image
8) Added "twitter:card" property to HTML_HEAD, setting to summary_large_image for gallery images
9) Documented Twitter Cards properly

I can see a potential for people wanting to use category images for the Open Graph image. For example, an Announcements image for an announcements news post. This has overhead, so isn't implemented by default. People can achieve it via templating though. They'd need to set the image metadata based on category ID.