You can add an image to your message by using the <img> tag. For example:
<img src='http://directpathoftheimagehere.com'>
The image is not technically inserted or embedded (as an in-line attachment) to your message. The <img> tag just creates a holding space for the referenced image.
To “source” an image, upload the image on your server (or any platform that stores images) and use the direct link/URL of the image as your image source (src).
Helpful resources: