i'm struggling to follow that code. as both hopen & \g are new to me.
but you are replacing the img alt tag with a title tag, I think ?
i picked out the chapter number with a regex and inserted it as text, in a small font, before the image. toc was OK then. not ideal but I can live with it.
so my code became
<h2 class="calibre10" id="heading_id_2">1 <img alt="[Chapter 1]" class="calibre11" src="../Images/001.jpg" /></h2>
and I set font size in calibre10 style to 0.5em
is img alt not supported in epub ?