Index: src/wp-admin/custom-header.php
===================================================================
--- src/wp-admin/custom-header.php	(revision 33851)
+++ src/wp-admin/custom-header.php	(working copy)
@@ -684,7 +684,10 @@
 	public function step_2() {
 		check_admin_referer('custom-header-upload', '_wpnonce-custom-header-upload');
 		if ( ! current_theme_supports( 'custom-header', 'uploads' ) )
-			wp_die( __( 'Cheatin’ uh?' ), 403 );
+			wp_die(
+				'<h1>' . __( 'Cheatin&#8217; uh?' ) . '</h1>' .
+				'<p>' . __( 'The current theme does not support uploading a custom header image.' ) . '</p>',
+				403 );
 
 		if ( empty( $_POST ) && isset( $_GET['file'] ) ) {
 			$attachment_id = absint( $_GET['file'] );
@@ -835,10 +838,16 @@
 		check_admin_referer( 'custom-header-crop-image' );
 
 		if ( ! current_theme_supports( 'custom-header', 'uploads' ) )
-			wp_die( __( 'Cheatin&#8217; uh?' ), 403 );
+			wp_die(
+				'<h1>' . __( 'Cheatin&#8217; uh?' ) . '</h1>' .
+				'<p>' . __( 'The current theme does not support uploading a custom header image.' ) . '</p>',
+				403 );
 
 		if ( ! empty( $_POST['skip-cropping'] ) && ! ( current_theme_supports( 'custom-header', 'flex-height' ) || current_theme_supports( 'custom-header', 'flex-width' ) ) )
-			wp_die( __( 'Cheatin&#8217; uh?' ), 403 );
+			wp_die(
+				'<h1>' . __( 'Cheatin&#8217; uh?' ) . '</h1>' .
+				'<p>' . __( 'The current theme does not support a flexible sized header image.' ) . '</p>',
+				403 );
 
 		if ( $_POST['oitar'] > 1 ) {
 			$_POST['x1'] = $_POST['x1'] * $_POST['oitar'];