Use jQuery Animate, try:
$(function () {
// Cache nav for better performance
$nav = $('#nav');
// Same for thumbnails
$thumbnails = $('.image_thumbnails');
$nav.on('mouseenter', function () {
// Use stop to clear the animation queue and jump to the end
$nav.stop().animate({
height: "84"
}, "slow");
$thumbnails.stop().animate({
opacity: 1
}, "slow");
});
$nav.on('mouseleave', function () {
$nav.stop().animate({
height: "8"
}, "slow");
$thumbnails.stop().animate({
opacity: 0
}, "slow");
});
});