Change 1000+ of WordPress Featured Images in 5 Minutes! › Forums › Questions and Answers › Broken Thumbnails › Reply To: Broken Thumbnails
Hi David, thank you for the credentials. The bug is in the browser Firefox. The thumbnails are displayed in the browsers Safari, Opera, Internet Explorer 11 and the new browser called CLIQZ.
WordPress supports the srcset attribute since version 4.4 to offer a “built-in” technique for responsive images. If the page is called via https but the images via http the broken image is shown. It works correctly if the protocol is the same for both (the page and the embedded images).
So the solution are to change the browser (not pretty), to wait for a fix in Firefox (not pretty, too), to change https to http on your server (not safe) or to insert some code in your installation as suggested in How do I disable responsive images in WP 4.4?.