your sample has a bug

[laszewsk]

your epub sample you generated has a bug. THis happens for example if the images can not be found

[weitblick]

Your information is not detailed enough to reproduce the error.
cover.jpg is mandatory
If other images are missing and still referenced with {{< image >}}, there is no error.
And why should the images not be found?

[laszewsk]

I created the make file so it's easily reproducable.

a) when running make
and then open the epub, you will see on my system that the test image 1-1 and test image 1-2 do not show in the epub
The typical blue icon with question mark shows up indicating that the images can not be found

[weitblick]

I don't understand your makefile. There is a bug in line 13/14 and in my opinion it's not a good strategie to create the example site.
Apart from that, when I go your way, the pictures are displayed.
Look at README.md: create a new Hugo site, clone the theme and copy everything of exampleSite/ in the project root folder (content, static/OEBPS, config.toml, deploy.sh)
if you send me the ebook.epub file, maybe I can see where the problem is.