In most of the cases, the picture of any GOD OR GODESS is identified by its symbol or lanchhan, for example lord Vishnu is which sudarshan chakra, Laxmi devi is on lotus,sarashwati devi with gitar similar is the case with Jain Tirthankars there are exeptions like lord Hanuman and lord Ganesh. kalika devi . However lord shiva's picture is identical and every where you will find alike.Now the question is HOW COME LORD SHIVA'S PICTURE OR FACE OF THE PICTURE IS MORE OR LESS UNIFORM AND WHY NOT OTHER GOD?
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More or less all Hindu gods look alike except for minor changes. the real question is how did those faces come.I think quit a lot of it is due to rja ravi verma whose paintings caught the imagination of every body and almost became a standard. the pictures you refer to like Goddess Saraswati with veena and Goddess Laxmi on lotus are some of the most famous.