Default photo for specific category when adding ad

If you mean a custom image loaded on [adverts_list] then i am afraid it’s not really possible without modifying original files.

In very least you would need to create a child template for file wpadverts/templates/list-item.php (as explained here https://wpadverts.com/documentation/child-themes-and-templates/ ) and modify this file to display some custom image if user did not upload any images.