Assuming the OPer has moved on after 2 years, this is for anyone looking for similar information.
<?php
// get selected people
$people = get_field('service_expert');
if ($people) {
$person = array_rand($people);
// the rest of this would depend on what type of field it is
// I'm assuming a relationship field
// and that you are returning a array of post object
get_the_post_thumbnail($person->ID, 'person-thumb');
}
?>
