We too have observed that if there is a picture inserted in-line in the mail`s body (or can be in a signature), then in Smartermail v16 WebMail, the picture will be missing and instead a broken image pictogram is shown. The image is instead available as an attachment.
It does not matter if the image is inserted using the WebMail interface itself or an email client like Outlook - in all other mail clients (including Outlook, Outlook.com`s WebMail, GMail`s WebMail) the picture shows up fine in-line in the mail body, while in SM16 WebMail it`s not.