... and if you want the text in the NCX file to be different than in the XHTML, you can use the title attribute. This works for images too:
<h3 title="1. The Beginning"
>Chapter 1: The Beginning</h3>
<h3 title="Chapter X"
><img src="chapter-10.jpg" alt="chapter 10"/></h3>
or so I believe, since I don't use Sigil