Is it legal to show images of mutilated fetuses on posters on a city street?

Today at lunchtime I walked out of my office in Cleveland to find a dozen teenagers standing by body height posters of mutilated aborted fetuses. Their parents were driving trucks with the same color images (larger) on the side of the trucks. Is it legal for individuals to do this—to display shocking and violent images in public? Is so, why is this? What if children were passing by? If it is legal, by the same token could I stand on the corner with giant photos of decapitated American hostages or the battered bodies of rape victims?