Google Chat: how to post with picture file using mostly javascript in Google Apps Script?

Question

I am working in Google Chat. I can post text all day long and would like to include a picture file. Google gives an example of an attachment using JSON, but I am looking for javascript to include a pic within the post.

var Body = '..'

// Pic file name in G Drive.
var Picture = DriveApp.getFilesByName('DMS.png');

// I know this needs to be JSON
const payload = JSON.stringify({ text: Body,
                                 imgInput: Picture });
const options = {
    method: 'POST',
    contentType: 'application/json',
    payload: payload,
};
UrlFetchApp.fetch(GOOGLE_CHAT_WEBHOOK_LINK, options);