“A holy man, perhaps Guru Nanak, approx. 1770-1800.” (source: The Sikh Foundation)
According to the Sikh Foundation, the Satinder Kaur Kapany Gallery at the Asian Art Museum in San Francisco, California, is “the only permanent collection of Sikh Art in a North American museum.” Due to the delicate nature of the collection, the pieces in the gallery are rotated every six months.
The Sikh Foundation provides a sample of the artwork on display during this spring, including the above portrait that is believed to be of Guru Nanak (the founder and first Guru of the Sikh faith), painted 300 years after his death. Other pieces also provide interesting views back in time.
More about the Asian Art Museum is available here.
americanturban.com 
It also opens the thought in me that if the Gurus Sahiban had wanted us to have portraits of them, they themselves would have had them painted.