Tried a different approach but still no luck:
The documentation indicates I can create a new image like this:
let image = new Image({image: '/Users/giles/Desktop/icon.png'})
and images have an image property containing ImageData:
log(image.image.type); // ImageData
The override value is also ImageData:
log(override.value.type); // ImageData
But this doesn't work?
override.value = image.image;