Checked.gif / Unchecked.gif images in emails are always served as http instead of https

[ElBafo]

Describe the bug

If you sent an email from a website with SSL certificate, the checked.gif / unchecked.gif are always sent as http://. In our case, the website is highly secured behind firewalls, WAF and you name it so the recipients doesn't see the checkboxes in their email.

To Reproduce

Steps to reproduce the behavior:

1. Add a true/false type field in a website with SSL
2. Submit the contact form
3. Receive the email
4. Inspect the email code

Expected behavior

Images should be delivered as https://..../images rather than http://...../images