This billboard appeared in Harrisburg, PA as a specific response to the “Year of the Bible” Resolution passed by Pennsylvania’s House in January. It was criticized (even by other atheists) for being unnecessarily shocking and racist.
I think it’s perfectly acceptable to be shocking and offensive in the defense of the truth. And I don’t think it’s racist to publish a concrete image to anchor the abstract concept of slavery.
If you’re interested in criticizing something for being unnecessarily shocking and racist, you should look at where the finger is pointing rather than staring at the finger.