Consider this image from the first Selma to Montgomery march in 1965 (source)
That's Amelia Boynton, left unconscious after being attacked on the Edmund Pettus Bridge on Bloody Sunday. To see an unarmed woman being treated like this helped to galvanize the Civil Rights movement and would lead to passage of the Voting Rights Act.
Images sometimes evoke pathos that mere prose cannot. The video of George Floyd is hard to watch, especially knowing he dies as a result. Likewise, the body of 3-year-old Alan Kurdi evoked deep emotions that such a thing could happen. It helps drive the electorate to take action, or at the very least pay attention to an issue.