No src and sizes attributes present on WordPress thumbnail images; the smallest image size is loaded irregardless of viewport size
WordPress Responsive Images Without Imagick No, you do not need Imagick to get WordPress responsive images (srcset and sizes) working. As of WordPress 4.4+, if your theme has post-thumbnails enabled and you are using core functions like the_post_thumbnail() or wp_get_attachment_image(), WordPress will automatically include srcset and sizes attributes (assuming multiple image sizes exist). The GD … Read more