If I understand your question correctly you want to know which value you need to place in the Google Maps widget. The Enum key is the name (not the caption) of your enumeration item. The name is the key that will be stored in the database and the caption is translatable for each language in your project.
The image is retrieved from an image collection which is a resource item of the menu in the project explorer: https://docs.mendix.com/refguide6/images