Index: image.php
===================================================================
--- image.php	(revision 46421)
+++ image.php	(working copy)
@@ -399,6 +399,18 @@
 
 	$new_sizes = array_filter( array_merge( $priority, $new_sizes ) );
 
+	/**
+	 * Fires before creating image sub-sizes.
+	 *
+	 * @since 5.3.0
+	 *
+	 * @param array  $new_sizes     Array defining what sizes to create.
+	 * @param string $file          Full path to the image file.
+	 * @param array  $image_meta    The attachment meta data array.
+	 * @param int    $attachment_id Attachment Id to process.
+	 */
+	do_action( 'wp_before_make_subsizes', $new_sizes, $file, $image_meta, $attachment_id );
+
 	$editor = wp_get_image_editor( $file );
 
 	if ( is_wp_error( $editor ) ) {