Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 35856)
+++ wp-includes/media.php	(working copy)
@@ -1286,6 +1286,15 @@
 		$image_base_url .= trailingslashit( $dirname );
 	}
 
+	// set base url scheme to match image source for consistent comparisons
+	if ( strpos( $image_src, '//' ) === 0 ) {
+		// protocol-relative URL, no protocol scheme
+		$image_base_url = preg_replace( '#^https?://#i', '//', $image_base_url );
+	} else {
+		$scheme = parse_url( $image_src, PHP_URL_SCHEME );
+		$image_base_url = set_url_scheme( $image_base_url, $scheme );
+	}
+
 	$all_sizes = wp_list_pluck( $image_meta['sizes'], 'file' );
 
 	foreach ( $all_sizes as $key => $file ) {
@@ -1293,7 +1302,7 @@
 	}
 
 	// Add the original image.
-	$all_sizes[] = $base_url . $image_meta['file'];
+	$all_sizes[] = $image_base_url . $image_meta['file'];
 
 	// Bail early if the image src doesn't match any of the known image sizes.
 	if ( ! in_array( $image_src, $all_sizes ) ) {