Hi dbreezy,
welcome to the Apple (user-to-user) Community.
Re: "only some of my contacts have their photos shown on my Mac in the messages app"
The contacts without photos may not have a photo added to their card in your "Contacts" app on the Mac ... if so, adding one there may fix.
How to:
Create and manage contacts in Messages on Mac - Apple Support
(Eg: article also says:
"You may not be able to add or edit contacts if you’re restricted by communication limits in Screen Time".)
In the Contacts app:
Manage and delete contacts on your iPhone or iPad
Else:
If needed:
You might want to reach out to the Mac mini Support Team:
There's a "Get Support" link at the bottom of their website (link below)
Mac mini - Official Apple Support
All the best :-)