IMPORTANT: The way images are included in a compose window has changed. Images are now included as data URIs and not as references to parts of other messages or operating system files. This allows better interoperability with office packages such as MS Office or LibreOffice. Images linked from locations on the internet will no longer be downloaded and attached to the message automatically. This can be changed for each image individually via the Image Properties dialog or globally by setting the preference mail.compose.attach_http_images.