As I wrote before, I can reproduce most of @daphy’s problem, now working with macOS v10.15.1. This problem definitely did not exist before updating to Catalina.
A transaction with an image will not display the image in a screen view, only the blue squares with question marks:
But when a PDF is created, or an email is sent from the same screen view, images show properly:
When printed using the Print button, images do not show. This is from the operating system’s built-in Print preview:
The one point on which my experience differs from @daphy’s is when I right-click on a blue square and select Open Image in New Window. In my case, a new window opens and the correct image appears in it. I have never seen any error messages.
Not knowing how image display is implemented in Manager, I wonder whether there are issues with Flash or Java settings? Catalina tightened up lots of things with respect to security.