It's taken 13 years but I've finally taken Ted Davis' advice and revisited the "vanilla" jpg header I detailed in this blog post about creating jpgs from random data https://www.hellocatfood.com/create-jpgs-in-pure-data/
And guess what? The jpg header is incorrect! One day when I start blogging again (never) I will update it, but until then enjoy this jpg made with a correct header and an atan function.