I created a new folder and a test document.
I moved the two .png files from the /images/github/ directory into the new folder and ran the following command:
asciidoctor -a stylesheet=github.css -a stylesdir=/usr/local/bin/asciidoctor-stylesheet-factory/stylesheets test.adoc
When I view the document, the images (for bullet items) do not show. The bullets appear when I render without using a stylesheet, so my syntax is correct.