For me it was showing the image and the prompt, but the whole page was unstyled. But when I reloaded the page now, the css loaded also and the prompt is not shown.

I guess the web server was temporarily overwhelmed by traffic resulting in images (like for you) and css files (like for me) not being consistently served to all visitors.

If you click the image, it flips to reveal the prompt and other metadata.

The blog is running on a Mac Mini on a 1Gb/s uplink so when it gets hit with a front page HN post, it does creak a little but I try to be environmentally responsible with my technology (as much as possible) so I am not in a hurry to move it into a datacentre when currently it only pulls 15-25W from the wall during peak traffic.

Is it a prompt or accessibility description for screen readers?

The website labels it as a prompt, so probably a prompt

mildly amusing that the model was instructed to generate it in the style of a "wide angle film still" but it seems to have gone for a painting instead