While setting the media content in the Composer Online template, the different attachment-related collections are suggested for selection. 

Attached Content Document 

This collection pulls up all of the related Images that you uploaded as Files. This is done in the Files related list. Salesforce switches to the Files framework to allow you more flexibility with your uploaded information - here you can upload such image formats as jpg., png., etc.

Attachment

This collection pulls up images from the AscendixRE Image Viewer component. Apart from that, it can pull up images from the soon-to-be-deprecated "Note & Attachment" related list. Not all of the organizations switched to the "Files" related list to work with images, so this might be a good option if you are working in an older organization.

Note, Attachment, Google Doc and File and Note and Attachment 

These collections are deprecated components that Salesforce will eventually stop supporting. Therefore, data from these collections is not going to be pulled up into Composer templates.