This was the line:
httpStatic: '/opt/nodered/node-red/images/',
The reason why it appears that the full path isn't displayed is that the instance is running on a LoRaWAN gateway rather than a computer with hdd.
What I actually found is that there is a secondary folder set up elsewhere on the gateway which node-red is pointing to for the image. Not sure why but I replaced the image in the second location with the correct one and it worked. I'll diagnose later as to why the image source points differently to what is in the setting file.
