Graphics not displayed in Apple Mail when website hosted by Bluehost

Your digital certificates or the associated web server DNS settings for what are commonly user-tracking images are probably incorrect, either overtly, or per what Apple requires.

Mail balks at loading images with invalid HTTPS responses for embedded images.

The people that work for Apple have not particularly documented their requirements here, so you’ll be doing some investigation with working servers and embedded images, of certificate vendors and certificate chains that do or do not work, or (easier) sending the images as attachments in the mail and avoiding what is commonly used for user tracking. (When enabled, Private Relay gets involved with masking user tracking here, too.)

There are some previous discussions of this around here in ASC; of mail and DNS and certificates and embedded images.

AlexandraAP wrote:

I feel that we are the only ones with this problem and the main purpose of our website is to send invitations via email to our events so it is incredibly frustrating that graphics to promote our events are just a '?' when opened by Apple Mail App which a lot of people have.

You definitely aren't the only ones. The problem is that Apple Mail has very specific requirements for the secure certificate of the web site that hosts graphics. Those requirements aren't documented anywhere. If you are still using some old-school web host, and/or creating your own certificates, then it is unlikely that you'll ever get it working. In my own tests, I'm unable to find a web host that I can setup and configure where it doesn't work. But then I use popular, name-brand, off-the-shelf certificates or AWS. For people who complain about this problem, that approach seems to be a non-starter.

Another option would be to simply include the image data inside your e-mail. Again, easy solutions are non-starters in the modern world.