That extension still needs some work. Here's generally how it works. If you pass the following document:
= Document Title
:doctype: book
== First Chapter
.Image in First Chapter
image::figure-a.png[]
== Second Chapter
.Image in Second Chapter
image::figure-b.png[]
You will get
Test Document
First Chapter
[figure-a]
Figure 1.1 Image in First Chapter
Second Chapter
[figure-b]
Figure 1.2 Image in Second Chapter
The numbering is not correct for the second figure. It should be Figure 2.1. But clearly it's showing that it is trying to make a two-part number. The key line in the extension is this one:
el.caption = replaced_caption
It's possible to change the caption (e.g., Figure 1.1) for any titled node in the document tree.
Feel free to open an issue in the extensions lab to get help from the community in using or improving that extension, or replacing it with a better one.
Cheers,
-Dan