Custom field for image url

There is a filter you can use to override the markup output, I would use that to solve this issue, here is an example of code that you can customize to grab the image from a post meta field:

function schema_wp_set_default_image_12345( $json ) {
	
	// If image is already defiend,
	// return our $json array and do not do anything
	if ( isset($json['media']) && ! empty($json['media']) ) return $json;
	
	// There is no image defined in Schema array, 
	// set default ImageObject url, width, and height
	$json['media']['@type'] = 'ImageObject'; // schema type, do not chage this!
	$json['media']['url'] = 'http://default-image-url.png'; // set default image url
	$json['media']['width'] = 720; // set image width
	$json['media']['height'] = 400;	// set image height
	
	return $json;
}
add_filter('schema_json', 'schema_wp_set_default_image_12345');

Here is a link to the code gist.

I hope this helps.