Featured Image issue for Listings
Please go to Form Builder > Listings > Edit > Make sure Featured Image field is available in the Builder Active Section
Then the user will be able to add an image there and it will be shown on the listings search and ET Listings element.
Then you can see that your search result page is showing featured image fine at your site,
When a user does not upload the featured image from submitting listing page then the default feature image which you have set in the
Theme Options > Listing Settings > General > Default Placeholder Image >
When you upload the default image here it will come up to those listings which do not have any featured image assigned to them.