RE: The Sanctum has Refreshed, But what does that mean?

Sourcing means posting the link to where they came from. As you say you don't have to link the images directly, however, if you use one of the official GU servers (for cards or art), linking the root domain will show an error page, which in itself is useless for whoever is interested in the image source.

Regarding the 'fair doctrine' thing, I still think it makes sense if you adapt an existing image you could say something like 'Original image from xyz'.

Cheers mate, keep on posting good reads here, it's appreciated!