Hello and welcome to 4images forum.
Unfortunately not everything can be done by changing the templates. There are some parts of HTML code generated in the 4images core. The subcategories display is one of them.
This topic is probably what you were looking for.
As of template itself. We already have
a lot of free different templates.
The templates are .html files with basic HTML code. The only things that might be confusing at first are {..} tags that will be replaced with appropriate code. Because each tag has it's own content in each template, there would be a huge undertake to create a list of each and every tag and what it represent after the template was parsed.
Yes, I agree, lack of template documentation makes it difficult for the beginners, but 4images uses such basic template engine that is easy to learn.