There is a very famous 20th century American painter named Jasper Johns. I have included a link about him below in case that is who you meant.
The only person named John Jasper that I have ever heard of was an African American preacher who lived in Richmond, Virginia. Jasper was born a slave and lived the first 50 years of his life as a slave. He preached as a slave and continued preaching after emancipation. After emancipation, he formed a free black congregation and built a church for it. The black church was very important in the days after emancipation. It was one of the main ways in which freed African Americans tried to build themselves a community in the face of white hostility. It is for this reason that people like John Jasper are important in American history.
According to Wikipedia and as stated above, Jasper Johns is an artistborn 5/13/30 who paints and specializes in printmaking.