How do you have your Fastmail remote image security set up? You can find this setting at:
Settings>Mail>Preferences>Reading>Load remote images
Your choices are:
- Always load external content
- Load external contents from my contacts, otherwise ask
- Ask before loading any external content
If you set that Fastmail security setting to the 2nd or 3rd setting shown above, you will get a warning message containing a link similar to this:
Remote images blocked to protect your privacy. Load images
My guess is that you are either ignoring that warning message or some setting in your browser is hiding it from you.
Bill