Why is the primary image blurry on mobile but high res if I rotate my phone horizontal?...

I took a quick look because that sounds odd. They've made a mess of the article's image sizing - only 270px wide if the viewport is <768px. (That top image is 1024px wide on desktop.) There's so much wrong with that decision, regardless of the huge difference between 270 and 768, e.g. apparently not being aware of hardware pixel density on smartphones.