Beware taking Copilot answers as the truth!
It had given me a static URL to use which, before adjustment for my actual org URL was
https://[ORG URL].dynamics.com/Image/download.aspx?Entity=account&Attribute=crc2a_image&Id=
Copilot said to add the GUID of the image onto the end of the URL and change the text for the correct org URL.
What it took me ages to discover is that this URL has been copied from a blog article written by someone where the logical name of their table (Entity) is "account" and the logical name of the column was crc2a_image.
I put the name of my table, and the column name in, but it still didn't work.
From sheer curiosity and experimentation, I discovered that the GUID that was actually needed was the RECORD GUID not the Image GUID. This is column which has the same name as the table.
Now it works!