Putting featured image in page label(value)

I made a code (on localhost) that makes checkboxes for each page currently in WordPress. Now I get the page ID and title when someone submits the checkbox form.

Now I need it to display the page’s featured image as well but I canoot find out the exact code to put in the value for that, or if I must make a variable first.

I tried this code: $the_post_thumbnail(); but nothing displayed

foreach ($pages as $page){
        $a = $a + 1;
        echo "<input type='checkbox' name='$a' value='$page->post_title $page->ID)'>";
        echo "<label> $page->post_title </label>";
        echo "<br/>";
    }

Does anyone know what I am missing? Thanks in advance