Index: src/wp-includes/customize/class-wp-customize-header-image-control.php
===================================================================
--- src/wp-includes/customize/class-wp-customize-header-image-control.php	(revision 40400)
+++ src/wp-includes/customize/class-wp-customize-header-image-control.php	(working copy)
@@ -173,20 +173,20 @@
 			<p class="customizer-section-intro customize-control-description">
 				<?php
 				if ( current_theme_supports( 'custom-header', 'video' ) ) {
-					_e( 'While you can crop images to your liking after clicking <strong>Add new image</strong>, we recommend matching the size of your video.' );
+                    _e( 'Click <strong>“Add new image”</strong> to upload an image file from your computer. Your theme works best with an image that matches the size of your video — you’ll be able to crop your image once you upload it for a perfect fit.');
 				} elseif ( $width && $height ) {
 					/* translators: %s: header size in pixels */
-					printf( __( 'While you can crop images to your liking after clicking <strong>Add new image</strong>, your theme recommends a header size of %s pixels.' ),
+					printf( __( 'Click <strong>“Add new image”</strong> to upload an image file from your computer. Your theme works best with an image with a header size of %s pixels — you’ll be able to crop your image once you upload it for a perfect fit.' ),
 						sprintf( '<strong>%s &times; %s</strong>', $width, $height )
 					);
 				} elseif ( $width ) {
 					/* translators: %s: header width in pixels */
-					printf( __( 'While you can crop images to your liking after clicking <strong>Add new image</strong>, your theme recommends a header width of %s pixels.' ),
+					printf( __( 'Click <strong>“Add new image”</strong> to upload an image file from your computer. Your theme works best with an image with a header width of %s pixels — you’ll be able to crop your image once you upload it for a perfect fit.' ),
 						sprintf( '<strong>%s</strong>', $width )
 					);
 				} else {
 					/* translators: %s: header height in pixels */
-					printf( __( 'While you can crop images to your liking after clicking <strong>Add new image</strong>, your theme recommends a header height of %s pixels.' ),
+					printf( __( 'Click <strong>“Add new image”</strong> to upload an image file from your computer. Your theme works best with an image with a header height of %s pixels — you’ll be able to crop your image once you upload it for a perfect fit.' ),
 						sprintf( '<strong>%s</strong>', $height )
 					);
 				}