This should do what you need.
- In WP goto WooCommerce | Products | Attributes.
On the right is a list of your Attributes and their Terms.
- Immediately below the Attribute name are two links “edit” and “delete”.
Click “edit”.
- The next screen looks similar it lists all the Terms for the selected attribute.
Click “edit” under the term name.
- On the left click the button “Upload/Add Image”.
Hope this gets you what you need.
Best.
Tim