Смотрите теперь один заработал а несколько не работают,
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
div.content .thumbnail a img {
height:300px;
width:300px;
}
</style>
</head>
<body>
<div class="content thumbnail">
<!--<div class="item">
<div class="thumbnail">
<a href="tratata.html"></a>
</div>
</div>
</div>-->
<div class="thumbnail"><a href="http://google.com" target="_blank" onclick=""><img src="../img/pixel.gif" class="product-image" style="background-image: url(http://img2.1001golos.ru/ratings/60000/59882/pic2.jpg)"></a></div>
<div class="thumbnail"><a href="http://google.com" target="_blank" onclick=""><img src="../img/pixel.gif" class="product-image" style="background-image: url(http://img2.1001golos.ru/ratings/60000/59882/pic2.jpg)"></a></div>
<div class="thumbnail"><a href="http://google.com" target="_blank" onclick=""><img src="../img/pixel.gif" class="product-image" style="background-image: url(http://img2.1001golos.ru/ratings/60000/59882/pic2.jpg)"></a></div>
<script>
document.querySelectorAll('div.content .thumbnail a').innerHTML = '<div class="product-image img-item" style="background-image: url(http://img.clickonf5.org/it/7GoogleChromeExtensionsYouMustHave_A5E0/google_chrome_extensions.png) no-repeat;">Insert text</div>';
</script>
</body>
</html>