In this day and age, it's not just the photo that plays a role in activism. A photo is just one of the essential parts.
The invention of the spoken language, the invention of the written language, invention of paper and ink, printing press, telegraph, radio, and now the Internet and the World Wide Web; the speed of communications and the amount of information available to us is increasing at a rapid pace.
100 years ago, during WWI, and a couple of decades later during WWII, a journalist (or anyone with a camera) could take a photo, but it took days and perhaps weeks or months for it to spread. But now, cameras have become palm sized (and in some cases, thumb sized). Anyone in the world who is at the right place right time could take a photo and within a few hours (depending on how far he/she is away from an Internet connection) could upload it and showcase it to the world.
Now in terms of whether a photo has impact or not, consider that throughout history the great masses of any population can be swayed by a charismatic leader (whether good or evil) who uses propaganda images (sight) and stirring speeches (audio). Imagine if all the communications technology we have today was available to Hitler over 70 years ago. (Imagine if Hitler had used the Internet and Youtube back in 1936 to post his speech on the opening ceremony of the Olympics, which in the novel and movie 'Contact' was said to have been the first video broadcast that was powerful enough to leave our solar system.)
The other mediums of communications are written text, spoken audio and a combination of visual and audio sense that makes up video.
A well written set of words (be it in the form of an essay, speech, manifesto or other) in itself could potentially have enough power to impact the minds of many. Enhancing that set of words with images (photos, graphics, etc.) could increase the power and impact it has. And turning that into an audio visual representation would enhance it even more.