
The metafield "theme.image" lets you set a custom image for Sub Collections inside the collection template. This image is separate from the collection’s featured image used on the collection list page.
Name: Collection image
Namespace & key: "theme.image"
Type: Image (File)

It displays in the Sub collections section on the collection page template.
It does not affect the featured image shown on the collection list/grid page.

If "theme.image" has an image:
The theme will show the metafield image in Sub collections
It will override the default collection image for that section only
If it’s empty:
The theme will automatically fall back to the collection image set in the Collection manager, or the first product image in the collection (if no collection image is set).

No. This metafield is auto-detected by the theme. Just upload the image into the metafield and it will work.
Use metafield "theme.image" When you want different visuals for Sub collections without changing the main collection image used elsewhere.