<?php

add_filter( 'em_event_output_placeholder', 'em_event_image_alt', 100, 3 );
function em_event_image_alt( $replace, $EM_Event, $result ) {
	
	if ( $result === '#_EVENTIMAGECUSTOM' ) {
		
		$image_id = get_post_thumbnail_id( $EM_Event->post_id );

		if ( $image_id ) {
			$alt = get_post_meta( $image_id, '_wp_attachment_image_alt', true );

			$replace = wp_get_attachment_image(
				$image_id,
				'medium',
				false,
				array( 'alt' => $alt )
			);
		}
	}

	return $replace;
}