In the admin section, the mail template can be configured. The MailTemplateAdminController adds a limit to the size of the mail template of 10'000. This limit is most likely random, there is no reason behind the limit.
For most situation this is enough, however when trying to add an inline image with src=data and base64 encoded image data, the size is too small. In addition, the user does not see what the problem is, there is no error dialog, it just doest not save.
The solution is to set the limit to something much larger. It should be possible to add a some-100k image to the footer of the mail template.