is_tree()

I added this function to my functions.php:

<?php
    function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
    	global $post;         // load details about this page
    	$anc = get_post_ancestors( $post->ID );
    	foreach($anc as $ancestor) {
    		if(is_page() && $ancestor == $pid) {
                return true;
    		}
    	}
    	if(is_page()&&(is_page($pid)))
            return true;   // we're at the page or at a sub page
    	else
            return false;  // we're elsewhere
    };
?>

and I am executing it outside of the loop:

<?php
    if (is_tree('home')){
        echo '<img src = "' . $siteUrl . '/images/content_title_productivityOptimisation.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('partners')){
        echo '<img src = "' . $siteUrl . '/images/content_title_partner.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('blog')){
        echo '<img src = "' . $siteUrl . '/images/content_title_blog.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('contact')){
        echo '<img src = "' . $siteUrl . '/images/content_title_contact.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('productivity-tools')){
        echo '<img src = "' . $siteUrl . '/images/content_title_productivityTools.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('training-workshops')){
        echo '<img src = "' . $siteUrl . '/images/content_title_trainingWorkshops.jpg" alt = "Welcome Title" />';
    }
    elseif (is_tree('productivity-platforms')){
        echo '<img src = "' . $siteUrl . '/images/content_title_productivityPlatforms.jpg" alt = "Welcome Title" />';
    }else{
    }
?>

It is only working if is_page is true, rather than if it is a child of the parent identified in my if statements.