I don't think I agree that terrorism emerged largely after WWII, unless we're talking about some very specific definition I'm not really aware of, but sure I understand what you're saying.
You can trace the history of modern terror back to the Reign of Terror under Robespierre during the French Revolution, but these were heads of state. Prior to this you have isolated incidents like the Gunpowder Plot or the Boston Tea Party, but these aren't non-state incidents - they're examples of state actors inciting unrest from within.
Non-state terrorism (i.e. terrorist groups that are not officially affiliated with any nation-state) didn't really appear until after WWII, and international terrorism is even more recent. As we understand it today, terrorism is a reaction to modernity.
If you have a pattern of examples of non-state terrorism prior to WWII, that would be a place to start...
Spreading the images aides in spreading the fear and so on. But has the increase in media really actually been shown to improve and increase terrorist activity or is this just some hypothetical common sense thing you're throwing out there?
I wouldn't call it common sense, mainly because I don't like the phrase. I don't think it's intuitive that media has increased terrorism, and I also don't think you can draw a direct correlation. Furthermore, you can't prove that media and terrorism are intertwined because in order to do so you would need to subtract one or the other; but as our history has shown, media and non-state terrorism rose alongside one another, meaning we have significant evidence that they're related.
Given the typical assertions of cultural theory, it's easy to fall into the trap that everything is connected to everything else (which, even if it's an accurate statement, isn't a very helpful one). In the case of terrorism and the media, however, I would suggest that their unconscious, even automatic, convergence bespeaks their cultural relation.
The extent of our knowledge regarding terrorism's relation to the media probably didn't form until after 9/11, when the towers falling became one of the most powerful images in Western culture. I think that a lot of scholars are correct when they say that the terrorists weren't only interested in physical violence (although this was part of it) - they were interested in creating an image that would haunt media culture for years (as it has).