Images can be uploaded directly to Media gallery, or uploaded while inserting in a page or post. The system will save multiple sizes and allow some simple manipulations such as rotation and cropping. Some of the existing images are also linked directly without necessarily being in the Media gallery.
Posts include articles. Could be News as on many sites, but then we would want to handle the articles differently.
Pages (special type of post), contains all the pages. Each has a sidebar selection at right. In addition to the typical formatting choices at the top of this content field, there is an assist for creating columns. I suggest look at one of the pages that has columns, and look for the “shortcode” at the beginning and end of each column to see the result. Then in a new page, click the desired columns from the icon above and follow its instructions.
Sidebars can be provided by the theme or can be created custom and are located at Appearance Widgets. Not all standard sidebars are used, only the primary. Custom sidebars are created under Genesis Simple Sidebars, then populated under Appearance Widgets, then selected on each page. These are essentially a way to create a side column on a page and populate it with some shared or individual material.
Testimonials can be populated under Testimonials post type, but I haven’t fully developed it.