Attachments can be used as embedded images in the HTML body. To use this feature, you need to set additional property of the attachment - cid (unique identifier of the file) which is a reference to the attachment file. The same cid value must be used as the image URL in HTML (using cid: as the URL protocol, see example below).
let message = {
...
html: 'Embedded image: <img src="cid:unique@nodemailer.com"/>',
attachments: [{
filename: 'image.png',
path: '/path/to/file',
cid: 'unique@nodemailer.com' //same cid value as in the html img src
}]
}