Super simple change that allows the calendar event API to output the cover photo... It's already being assigned, just doesn't seem to be being set in the output.

public function apiOutput( \IPS\Member $authorizedMember = NULL )
	{
		return array(
			'id'			=> $this->id,
			'title'			=> $this->title,
			'calendar'		=> $this->container()->apiOutput( $authorizedMember ),
			'start'			=> $this->_start_date->rfc3339(),
			'end'			=> $this->_end_date ? $this->_end_date->rfc3339() : NULL,
			'recurrence'	=> $this->recurring,
			'rsvp'			=> (bool) $this->rsvp,
			'rsvpLimit'		=> $this->rsvp_limit == -1 ? NULL : $this->rsvp_limit,
			'location'		=> $this->_location ? $this->_location->apiOutput( $authorizedMember ) : NULL,
			'venue'			=> $this->venue() ? $this->venue()->apiOutput( $authorizedMember ) : NULL,
			'author'		=> $this->author()->apiOutput( $authorizedMember ),
			'postedDate'	=> \IPS\DateTime::ts( $this->saved )->rfc3339(),
			'description'	=> $this->content(),
			'comments'		=> $this->comments,
			'reviews'		=> $this->reviews,
			'views'			=> $this->views,
			'prefix'		=> $this->prefix(),
			'tags'			=> $this->tags(),
			'locked'		=> (bool) $this->locked(),
			'hidden'		=> (bool) $this->hidden(),
			'featured'		=> (bool) $this->mapped('featured'),
			'url'			=> (string) $this->url(),
			'cover_photo'   => (string) $this->cover_photo
		);
	}

/applications/calendar/sources/Event/Event.php

Last line there is all that would need to be added to 

You can extend that method and add the column in the API.