The extended image extends past the content of the page or post. By default, the text on a page only stretches part of the way off the page. That way, it makes paragraphs of text easier to read, and images stand out. After all, a picture is worth 1000 words.
A full width image breaks out of the box. It can be created by adding a full-width slideshow to your page or post, and only adding a single slide.
A parallax image stays fixed to the background, and the page content scrolls around it. Perfect for a big reveal. The user is rewarded for scrolling and interacting with your site, and they can see a full-screen image – a piece at a time.