Hi David,
There’re two scenarios:
If you are using a container with a dynamic background image set to the featured image, you just need to upload an actual image, it will be the fallback image if there’s no featured image.
If you are using a GB image block to pull the featured image, you can add one more image block with the fallback image below it, add a CSS class to the dynamic GB image block, eg. featured-image
, so we can use CSS to target it.
Here’s the block structure for your reference, the red one is the dynamic image, and the green one is the static fallback image:
https://www.screencast.com/t/ry7zgkE4
Here’s the CSS:
.gb-query-loop-item .gb-block-image:has(.featured-image) + .gb-block-image {
display: none;
}