@dmmarti It was simpler than that - I'm a dumdum and I had so many folders with CoCo BIOS files in them from when I was trying to get it work that I'd added the Extended Basic ROMs to every folder except the right one :D
Copy the PNG to a directory accessible to the pi user (like ~/RetroPie/ROMs/coco/media/images/
From ES, highlight the folder that you wish to display the image
Hit your "Select" button
Select "Edit this game's metadata"
Type the path where the image is
For number 6, I can't remember if you can use relative paths or even the tilde (~) so probably best to use the full path. So for step 1 above, the path would be: