Currently, email notifications do not include any images or other attachments. Instead, an empty box is shown with a question mark. This is confusing and visually unattractive. Besides, it gives end users the impression that Confluence is not working properly.

People have been complaining about this for years, both in the Confluence and JIRA forums. See e.g. this Confluence thread and this JIRA thread

So why not add an option to the mail server configuration settings under Administration > General Configuration > Mail Servers, to strip all images / attachments from email notifications? This would be an elegant solution for end users (who would rather see no images at all than broken images) and also means no complex solution has to be built for handling permissions to download / view images from restricted Confluence posts or pages.