James, please focus on what I am asking you to do.
There are two completely unrelated tasks I want you to complete.
In Family Historian, please click the big blue arrow shown in your earlier screenshot.
In the Multimedia Object Property Box, what does it say in the box labelled File: near the top?
These instructions do NOT involve running the Family Historian program.
Now it is very important to carry out these instructions otherwise the problem will not get fixed.
Open File Explorer by double-clicking This PC on your desktop. i.e. the application that displays all the folders and files on your disk drive.
Navigate into the Documents folder and then the Family Historian Projects folder.
Now open the folder that matches your Project name.
Now open the folder that matches your Project name but with the suffix .fh_data
Now open the folder called Cache
Delete the ThumbCache.dat file.
Does that cure the problem?