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.<p>e.g. For Climate Change they're showing the header for <a href="https://medium.com/invironment/an-army-of-ocean-farmers-on-the-frontlines-of-the-blue-green-economic-revolution-d5ae171285a3" rel="nofollow">https://medium.com/invironment/an-army-of-ocean-farmers-on-t...</a>