For example, https://payments.wikimedia.org/extensions/DonationInterface/gateway_forms/includes/card-visa.png
Response headers are,
Accept-Ranges: bytes Connection: keep-alive Content-Length: 19112 Content-Type: image/png Date: Thu, 01 Oct 2015 19:19:50 GMT Etag: "241706-4aa8-50931bfc59dc0" Last-Modified: Tue, 02 Dec 2014 01:35:59 GMT Server: nginx strict-transport-security: max-age=15552000
Without a cache-control header, the browser has to make a request for all the images and wait for the HTTP 304 response when the page is reloaded.