Also, just to warn: spaces in filenames aren't a good idea.
So if you adjust DNSB's to:
Replace: <img alt="Fig. \1" class="whatever"
src="../Images/Fig\1.jpg"/>
that would get you:
Code:
<img alt="Fig. 2.4" class="whatever" src="../Images/Fig2.4.jpg"/>
instead.
I would then recommend using Sigil's mass renaming to fix the filenames.
Follow steps 3 and 4 in this little tutorial I wrote back in 2019.