Limit Slider Revolution's WP_Query to only include posts with featured images

functions.php

<?php

/**
 * Limit Slider Revolution's WP_Query to only include posts with featured images
 *
 * Based on: https://gist.github.com/cliffordp/9f0d7cc5e86b2a721cd646d953bb1261
 * Similar code for Essential Grid: https://gist.github.com/Camwyn/cc191bc1c95581c38f7ed5f8ba61da11
 * Same Slider Revolution code except with additional requirement to be a Featured Event from The Events Calendar: https://gist.github.com/Camwyn/ce34677cafe52d46c12c11c8b9119371
 *
 * Tested working with version 6.5.24
 *
 * `revslider_get_posts` filter is from plugins/revslider/includes/slider.class.php as of version 6.5.24
 *
 * Filter hook only applies to Slider Revolution > Post-Based Slider > Fetch Posts By Categories & Tags
 * or Slider Revolution > Post-Based Slider > Specific Posts > Specific Posts List (CSV of Post IDs)
 * There are others in plugins/revslider/includes/slider.class.php for other queries.
 *
 * @return array
 */
function revslider_post_based_require_featured_image( $query, $slider_id ) {

	/* YOU MUST CHANGE THESE TO YOUR OWN SLIDER REVOLUTION SLIDER IDs!!! */
	$slider_ids_to_affect = array( 2, 20, 34 ); // include IDs of sliders to affect

	// If this slider is not one to affect, do no filtering.
	if ( ! in_array( $slider_id, $slider_ids_to_affect ) ) {
		return $query;
	}

	// Get the existing meta_query so we aren't wiping that out.
	if ( ! empty( $query['meta_query'] ) ) {
		$meta_query = (array) $query[ 'meta_query' ];
	} else {
		$meta_query = [];
	}

	// do the filtering...

	// Restrict to events(posts) that have a featured image.
	$meta_query[] = [
		'compare' => 'BETWEEN',
		'key'     => '_thumbnail_id',
		'value'   => [ 1, PHP_INT_MAX ],
	];

	$query['meta_query'] = $meta_query;

	return $query;
}

add_filter( 'revslider_get_posts', 'revslider_post_based_require_featured_image', 10, 2 );

Note: the original of this used 'compare' => 'EXISTS' which would fail often as a missing image can be saved in the database as a NULL value - thus it "exists".

Instead we test that it's a number between 1 (skipping post 0 which is typically a demo post) and PHP_INT_MAX