Whoever picked Mediums category images did a horrible job (i.imgur.com)

Sorry couldn't post the link, it was part of the sign-up wizard.

This appears to be header images taken from popular or random recent articles within that category, rather than an image specifically chosen to represent the entire category.

e.g. For Climate Change they're showing the header for https://medium.com/invironment/an-army-of-ocean-farmers-on-t...