If you delete the image from the web server (or rename it), the signature will show a placeholder and error.The image may download every time they view the message, which could increase their bandwidth costs. If you link to an image, users could block external content, so they won't see the image and they won't see the image if they are not connected to the Internet.However, they'll only download the image once, which makes it bandwidth-friendly. If you use an embedded image, the message size will be larger (because of the attached image) but the recipient will see the image even if they are offline or when they look at the message years from now.
