Jump back a few hundred years. When someone wanted to make a point to a large audience, he would often do it through well-articulated text: religious documents such as Thomas Aquinas' Summa Theologica and Martin Luther's 95 theses; philosophical works such as Kant's Critiques or political documents like the Declaration of Independence, Paine's Common Sense or Marx's Communist Manifesto. Each of these had a profound influence, and they were each delivered through well-written text.
Now jump to today--the TV, Internet and YouTube era. Instead of publishing a dense, articulate theological treatise, pastors who want to make a point will give a riveting, emotional speech--often in a megachurch auditorium, over TV, the radio, or online videos. Politicians do the same thing. In addition, more and more people are satisfied with staying up to date with politics through watching the Colbert report rather than reading political articles.
I think that at least two major factors are leading people away from books and more towards videos and images: 1) deep down, I believe that humans are emotional beings--we respond deeply to emotions; and 2) like electricity, we tend to take the path with least resistance (we find the closest parking spot in mall parking lots, we'll go through a door that's already open instead of opening the one next to it, etc).
It's requires less effort to get an emotional response from watching a funny or emotional video than to read a whole book or an article. Even if the reading is more enlightening, empowering, and invokes a deeper emotional response, it ultimately requires more time and effort--it's pulling open the closed door rather than walking through the nearby open door, like the funny TV show or YouTube video.
People will still continue to read and write--a lot. However, due to the quick access to reading and writing, I think the quality of day-to-day text will decline. I read that people in Shakespeare's day commanded a vocabulary that is nearly 10 times what we use in speech today--and our common vocabulary today is declining further through texts, email, and Twitter jargon (gr8, bff, lol, etc).
The bottom line--Images and videos tend to invoke emotional responses more quickly than books do; I think written text is undergoing a slow erosion, and is being replaced in many ways, ironically, by spoken text (via film, TV, the Internet) and quickly-typed writing we use in texts, emails, social networks and blogs. Books are still booming (ie: Harry Potter, Twilight (unfortunately), and their derivatives), and will always be around--whether in print or through digital books--but I think their influence is slowly being overshadowed by the more instantaneous sources.