Seeing the Images have unrecognisable generic names

Make them meaningful (eg. ..\Images\chapter_1), hopefully using the great Bulk renamer Meme built.
(Sigil adjusts the references )
Now that is done,
REGEX time
Code fragments
your search term: will include:
<h1 class="calibre8" id="calibre_pb_(\d+)">
Images\\chapter (\d+) <- note the SECOND forward reference
You are going to replace
<h1 class="calibre8" title="
Chapter \2" > The Bold is what will be in the TOC
and repair the captured text
Images\chapter \2
I just threw away the id tag as Sigil's TOC generator will make a new one if missing