On remarquera que ce qui diffère à chaque code est la valeur "box-shadow".
-10px -10px
10px -10px
10px 10px
-10px 10px
L'image éditée a pour adresse
http://img.over-blog-kiwi.com/1/30/51/51/20150805/ob_870e9d_11.jpg
<img src="http://img.over-blog-kiwi.com/1/30/51/51/20150805/ob_870e9d_11.jpg" alt="exemple css box-shadow" style="height:267px; box-shadow:-10px -10px green; margin:10px;" />
<img src="http://img.over-blog-kiwi.com/1/30/51/51/20150805/ob_870e9d_11.jpg" alt="exemple css box-shadow" style="height:267px; box-shadow:10px -10px green; margin:10px;" />
<img src="http://img.over-blog-kiwi.com/1/30/51/51/20150805/ob_870e9d_11.jpg" alt="exemple css box-shadow" style="height:267px; box-shadow:10px 10px green; margin:10px;" />
<img src="http://img.over-blog-kiwi.com/1/30/51/51/20150805/ob_870e9d_11.jpg" alt="exemple css box-shadow" style="height:267px; box-shadow:-10px 10px green; margin:10px;" />