On the 'Components' tag on the Drag & Drop editor, drag an image block onto the message.
Click on the 'Select Image' button.
Either upload an image from your computer by dragging the file to the modal then double-clicking to use it or clicking the "Use image from URL" button if you uploaded your image elsewhere.
If you chose to click "Use image from URL", the box below will appear. Simply type or paste the link where your image is uploaded and click "Use this image".
Please ensure that the image's url is https and not http, chrome (at least) does not want to load non https images.
Your image should now replace the initial placeholder.