Read the unit in order the first time. It starts where every template starts, with an image area and the object it gives you: what an empty area returns, the test that tells it apart from a real image, and the placeholder setting behind grey boxes on production. It then covers what the object tells you, how to resize, convert and render it in the order that fails safely and caches once, and what each resize strategy does.
The second half follows the image out of the template: the same transformations as URL parameters for scripts, emails and headless front ends, including where a hand-written URL behaves differently from the object; the picture tag the platform can generate; compression and the image services; and a closing reference of every image setting with its default. Come back to that last page when the same template renders images differently on two sites.
The unit assumes you know how templates read areas, from the units on rendering areas and on the shorthand.