This About/Map Tools feature uses the List widget, and requires image URLs and a spreadsheet.
Step One - Create a spreadsheet
The spreadsheet needs three columns, one for the image URL, title, and description (the title or description is optional). Fill out the titles and/or descriptions.
Step Two - Add images to AGOL
Follow this blog post to add your images to AGOL, and copy the image URL to the spreadsheet. Your spreadsheet should look something like this:
Step Three - Publish spreadsheet as table to AGOL
In AGOL, go to Content, add new item, and add the spreadsheet and create a hosted table.
