I have recently got this stamps from Postmuse, and they immediately caught my attention (besides their beauty). Especially the one in the centre.
I look for some information and I found that it was issued on August 23rd, 2013 to commemorate the 50th anniversary of the March on Washington. On August 28, 1963, more than 250,000 people participated in a march on Washington to promote jobs and equal rights. It was an inspirational demonstration for freedom. The highlight was the Martin Luther King Jr.'s "I Have a Dream" speech. The stamps shows marchers against the background of the Washington Monument.
Additional information about the stamp can be find here and here.