The banner image has a maximum width of 768. This isn't quite right when viewing an article on an iPad in landscape, say.
This bug was originally reported by @KHammerstein.
"The width of the image should match the width of the content, with a minimum width of 768 - so that its fills the screen in iPad portrait mode. But I forgot that in landscape mode the content stretches to 893, so the maximum image width should be 893 also."