.hoverthumb {
    position: absolute;
    border: 1px solid #ccc;
    background: #333;
    padding: 5px;
    display: none;
    color: #fff;
}

jQuery(document).ready(function($) {

    xOffset = 10;
    yOffset = 30;

    $("a.thumbnail").hover(function(e) {
        this.t = this.title;
        this.title = "";
        var c = (this.t != "") ? "<br/>" + this.t : "";
        $("body").append("<p class='hoverthumb'><img src='" + this.rel + "' />" + c + "</p>");
        $(".hoverthumb").css("top", (e.pageY - xOffset) + "px").css("left", (e.pageX + yOffset) + "px").fadeIn("fast");
    },

    function() {
        $(".hoverthumb").remove();
    });

    $("a.thumbnail").mousemove(function(e) {
        $(".hoverthumb").css("top", (e.pageY - xOffset) + "px").css("left", (e.pageX + yOffset) + "px");
    });
});

<?php

function custom_hover_scripts() {
	wp_enqueue_script('jquery');
	wp_enqueue_script('jquery_hover', get_stylesheet_directory_uri() . '/js/hover.js');
	wp_enqueue_style('jquery_hover', get_stylesheet_directory_uri() . '/css/hover.css');
}

add_action('wp_enqueue_scripts','custom_hover_scripts');

function hover_thumbnail($atts,$content) {
	global $post;
	$thumbnail = wp_get_attachment_url(get_post_thumbnail_id($post->ID));
	$content = '<a href="'. get_permalink($post->ID) . '" class="thumbnail" rel="'. $thumbnail . '">'.get_the_title().'</a>';
	return $content;
}

add_shortcode('hoverthumb', 'hover_thumbnail');