Visual imagery is powerful, there is no arguing this. The levels and impact of that power vary, and can be manipulated to differing strength by the author. 

 Kelsey Weyerbacher's video essay on the topic of assault is powerful for several reasons - 

• The photograph - monochromatic, simple with dark paint marring the skin on the individuals, representing the assault and the mark it left. 
• The music - Kelsey chose a song ( I believe by Mumford & Sons) that was both melancholy and triumphant. That, coupled with the images send the message that despite the wrong that has been done, these people will go on, they will rise above. 
• The use of stark facts and data about assault rather than opinion or conjecture - this was a very concise, to the point make people aware of the gravity of what is happening. It is direct and difficult to ignore.

All these factors, combined with the authors obvious understanding of the topic, make for a highly effective and moving video essay.