How to show an image popup when one clicks on an image thumbnail?

Question

1) enque thickbox on front end

add_action('init', 'myplugin_thickbox');
function myplugin_thickbox() {
    if (! is_admin()) {
        wp_enqueue_script('thickbox', null,  array('jquery'));
        wp_enqueue_style('thickbox.css', "https://wordpress.stackexchange.com/".WPINC.'/js/thickbox/thickbox.css', null, '1.0');
    }
}

2) Add thickbox to all your images that are displayed by the_content();

function fb_add_thickbox($content){

    $content = preg_replace('/<a(.*?)href="https://wordpress.stackexchange.com/questions/106603/(.*?).(jpg"jpeg|png|gif|bmp|ico)"(.*?)><img/U', '<a$1href="$2.$3" $4 class="thickbox"><img', $content);

    return $content;
}
add_filter('the_content', 'fb_add_thickbox', 2);

sources:
http://wordpress.org/support/topic/how-to-implement-thickbox-to-all-images-which-contain-a-refered-link
http://www.ultimatewebtips.com/how-to-use-wordpress-native-thickbox-support-in-front-end/