Which template to change to new roles to Docbook elements for HTML output?

I would like to add additional classes to my epub3 output.  I looked at Bob's book (http://www.sagehill.net/docbookxsl/AddNewTemplate.html#AttValueFormatting) and from what I understand I have to pick an existing template. 

Which templates do I need to use if I want to change imageobject and sect1 tags?

Carlos