We were learning about him in my history class and if you look at the portrait of him on Wikipedia, he is wearing a necklace with a picture of what appears to be a white guy. I was wondering if anybody knows who that is and why he's wearing it.
It is a Peace Medal. These were medallions given to Native American leaders by Spanish, French, British and American indian agents during peace treaties or during other diplomatic encounters. For instance, Lewis and Clarke carried with them several medals depicting Thomas Jefferson, which they presented to the chiefs of the various societies they made first contact with during their voyage.
The portrait on the wikipedia page appears to depict the Andrew Jackson peace medal.
In Black Hawk's autobiography he mentions receiving medals from the British indian agent for Upper Canada, Robert Dickson, at the beginning of the War of 1812. He also mentions receiving a second medal from Dickson's (unnamed) replacement after the War of 1812, as recognition for the efforts of Black Hawk and his followers during the war. He does not describe the medals in any detail, but it might be that these medals depicted King George III, the British monarch until 1820. I will admit that I don't know very much about British practice, and can't say how standardized their medals were.
In his autobiography, written in 1833, Black Hawk does not mention receiving medals from the Americans. However, it may be that he received a medal or medals some time between 1833 and his death in 1838. An Afterword to his biography recounts an account from the Annals of Iowa 1863/1864 concerning his burial. That account states that he was buried with a John Quincy Adams peace medal, an Andrew Jackson peace medal, and a medal from the city of Boston.
It seems plausible that Black Hawk received a peace medal from the Jackson administration, because Black Hawk
took a tour of the eastern United States traveled to Washington DC to negotiate peace between Sauk and Sioux in 1837. During that tour, the artist Charles Bird King painted a portrait of Black Hawk, the portrait you saw on the Wikipedia article. This portrait would seem to give credibility to the idea that Black Hawk was given the peace medal at some point between 1833 and 1837, and likely during the 1837 tour.