So the code from Lix works just from part. But here is an full working code.
$externalImage = 'External link to image';
$tempImagePath = 'Local link to save image';
$user = 123456789; // USER ID
// save image
file_put_contents( $tempImagePath, file_get_contents( $externalImage ));
// Create data for new event
$data = array(
'name' => 'Event title',
'description' => 'Event description',
'owner' => $user, // user as owner of this event
'location' => 'Location',
'start_time' => '2013-05-08T11:00:00-0700',
'end_time' => '2013-05-09T19:00:00-0700',
'@file.jpg' => '@'.realpath($tempImagePath),
'privacy_type' => 'OPEN'
);
// Create event
$eventID = $facebook->api('/'.$account.'/events', 'post', $data);
If you have some questions just write me.