Imported listing now showing default image, but default CPT image

URL and login details please.
I think the issue is that you are telling the system which is the default category, but that category is not added to the list of categories. A listing cannot have a category as default if the listing is not in that category. But I would need to check, have a test yourself please.
Or the category does not exist for that CPT.

I had no problem importing the GUUST TEST listing:
* Make sure the default category is a category applicable for the listing.
* NULL is not a valid ID for a post_author.
* Are you using libreoffice?
* You should leave cells empty instead of entering NULL.

* Make sure the default category is a category applicable for the listing, not the CPT, so make sure it is in the post_category column before adding it to the default_category column.
* NULL is not a valid ID for a post_author[probably not the issue].
* Are you using libreoffice?

Have you tried yourself with correct values?

You can only choose a default category for a listing if it is a category for the listing. Simples.
You cannot categorise a car as a vehicle, and then say the default category is a book.

This is a basic example: Macca’s can be categorised as a takeaway, restaurant and coffeeshop. That is 3 categories. The default category MUST be one of those 3 categories [it cannot be something else like a hotel suddenly]