Why is this not working? For loop output

<?php
$post_id = $wpdb->get_var(“SELECT meta_value from wp_postmeta WHERE meta_key=’slideimage'”);
foreach ($post_id as $post) {

echo $post_id->meta_value
}

?>

It doesn’t output anything, when I check with print_r, this is what I get:
Array ( [0] => stdClass Object ( [meta_value] => slide143.png ) [1] => stdClass Object ( [meta_value] => slide2.png ) ) 1

I want basically out the value of the image file. The meta_value.