Try this instead :
//Related stock photo function
function custom_content_after_post($content){
if (get_post_type( get_the_ID() ) == 'portfolio') {
//Query for the the related posts - portfolio custom post type
$loop = new WP_Query( array('post_type' => 'portfolio', 'posts_per_page' => 5, 'orderby' => 'rand'));
$sptitle="<h4>Related Stock Photos</h4>";
while ( $loop->have_posts() ) {
$loop->the_post();
$div = '<div style="margin:25px 10px 0px 10px;">';
$div .= '<ul style="list-style-type:none;">';
$div .= '<li style="display:inline;float:left;">';
//Display post thumbnail assigned to it.
$div .= '<div style="margin:25px 10px 0px 10px;">';
$div .= '<a href="' .get_permalink(). '" >' .get_the_post_thumbnail($page->ID, 'thumbnail'). '</a>';
$div .= '<p><strong><a href="' .get_permalink(). '" >' . get_the_title() . '</a></strong></p>';
$div .= '</div>';
$div .= '</li>';
$div .= '</ul>';
$div .= '</div>';
}
}
return $content .$sptitle .$div;
}
add_filter( 'the_content', 'custom_content_after_post' );