I disagree. Users, not just maintainers, discover projects through GitHub. In this context, the README effectively serves as a secondary project landing page. IMO, for a visual project like this, images, or a direct link to images, is a must.
The idea is to help people understand the project. Not to give them an investigation task. I don’t know the structure of that web site or if there are even images there.
I’m not heartbroken because of this gap, I’m just confused why they wouldn’t want to add these docs to help users.