adding description to custom header images

start by creating a child theme; http://codex.ww.wp.xz.cn/Child_Themes

then save a copy of header.php into the child theme folder, and add this this code:
http://pastebin.com/VbjxyMRF

to be added before this line in header.php of the child theme:
<?php endif; // end check for removed header image ?>

use css to format the position and layout of the description text.

now I want to edit the description and I don’t find it…

under
dashboard – media – library: find the images marked with ‘header image’ in the second column; then click edit under the image name…