Some email services (like gmail) will occasionally block images in emails unless those images are stored on their server. (This is a security measure.) This is an ongoing source of frustration for me.
Additionally, you need to make sure that the image reference is a fully qualified URL. Email knows nothing about relative URLs.
You may need to convert your image to base64 code which essentially stores the image data completely in the body of the email... Take a look at the following for details:
www.base64-image.de/tutorial
In the distant future, I may add an option to convert image references in emails to base64, but I have not looked into that yet.
