Fallback Image for Dynamic Image from Post Meta

1. Add this PHP code to add a fallback image ID to the image:

add_filter( 'generateblocks_dynamic_image_fallback', 'custom_dynamic_image_fallback', 10, 3 );

function custom_dynamic_image_fallback( $featured_image_id, $attributes, $block ) {
    if ( isset( $attributes['className'] ) && strpos( $attributes['className'], 'my-image' ) !== false ) {
	$featured_image_id = get_post_thumbnail_id(get_the_ID());
        $image = get_post_meta(get_the_ID(), 'testimonial_photo', true);
        if ( $image ) {
            // return the $image
            return $image;
        }
    }

    return $featured_image_id;
}

Adding PHP: https://docs.generatepress.com/article/adding-php/

2. Add my-image class to the image block.
Adding CSS class(es): https://wordpress.com/support/wordpress-editor/adding-additional-css-classes-to-blocks/

You are welcome   🙂