Add file format option to public API

[querkmachine]

I follow Zach's approach of using the screenshot service to generate OpenGraph images, which works nicely (and is far simpler than my previous approaches).

However, my OpenGraph images are pretty simple, with simple colours, shapes, and illustrations. Unfortunately, that means the screenshot service's defaulting to JPEGs makes them look pretty crunchy, with plenty of visible artifacts and noise. It's not terrible, but it is noticeable.

Having the option to generate a PNG without needing to edit and deploy a customised version would be nice to have.