Mansoor and Eric found the answer to the question of my last post : I was in Chicago. The stamp that I have shown was issued on the 20th of February 1987 by US postal administration in the Black heritage series. It pictures Jean Baptiste Pointe du Sable (1745?-1818) who is known as "the father of Chicago". He was a Haitian colonnist and was the first non indigenous settler in what is now Chicago. He was officialy nominated Founder of Chicago in 1968!
So I spent few days in Chicago with no time to deal with stamps unfortunatly. I'm currently writing this post from Ottawa (Canada). I'm going back to France tonight.