Step 1: Click on message centre ---> Templates ---> Broadcast
Step 2: Click on the template that you want to use
Step 3: Click on edit
Step 4: Click on Insert Tab and then click on Insert Image
Step 5: Enter Image URL and click Ok. Please keep in mind that this only works for images that are hosted somewhere. Users are not able to upload their own images into the template directly, you will need to upload images to your site and then link it this way.
Step 6: Click Save once you are done and your broadcast message is ready with the image embedded