How can I add an image, additional text and a button to the default Thank You Page?

Your instructions for Setting up the Thank You Page say that it's possible to completely customise the look from the thank you message editor:  https://www.jotform.com/help/211-Setting-up-the-Thank-You-Page

To insert images, links, paragraphs and tables.

This refers to the Thank You Page being edited from the SETTINGS Menu.

Currently the Thank You Page is editable from the bottom of the BUILD Form, not the SETTINGS TAB, but it doesn't have the images, links and extra paragraphs features the user guide speaks of.

There is no thank you message editor above the big green tick at the bottom of the BUILD Form. There is a Thank You image Properties pop-up, but this is very limited to the icon editing and all I can do is change the Thank You message below.

Is the full Thank You Message Editor located somewhere else now? How do I edit /insert images, links, paragraphs?

Hello,

Our apologies for the confusion. We have two form layouts, the classic and the new one, which is the card form layout. You currently have the card layout in your form. 

The shared guide is for the classic layout. The thank you option in the card is located at the bottom part and is limited to just the heading and subheading and then the icon. 

The icon can be a custom image. Just please remove the icon first to upload an image. Unfortunately, there are no other formattings in the thank you of the card layout. You may consider creating a custom thank you on your website if you want to customize the message with different layouts.