This question is ambiguous, so I am going to discuss each possible interpretation and offer some assistance for each. First, the focus of your paper might be upon how terrorists use different modalities of mass media to create perceptions about them, and second, the focus of your paper might be how mass media create perceptions about terrorists. Either way, you are writing a paper about how perception is created.
Terrorist organizations certainly do use different modalities of mass media to create perception, including social networks, national and international newspapers, and radio and television. Some of them seem to have their own "marketing" department. Research on this will involve tracking the information or propaganda that is provided intentionally by terrorist organizations and then analyzing its impact upon public perception. A few examples of this come to mind. There are the dreadful videos of beheadings that have been released within the last year. There are social networking efforts, for example, on Facebook, meant to lure people into sympathizing and joining the terrorist organizations. Going back in time, to the beginning of the revelation of the magnitude and power of the terrorist movement, there was a release of a video of Osama bin Laden after the World Trade Center was destroyed. Terrorists have issued news releases about their intentions, too, their intention, for one, to restore the Caliphate of earlier times. One way to proceed with your research would be to find a good collection of examples in various modalities and then analyze their impact upon perception. You might find that polls taken before the release of the beheading videos found that few Americans wanted to risk the lives of Americans to fight terrorism, but polls taken after showed many Americans willing to take that risk. Similarly, you might find that many more politicians were making speeches about fighting terrorism after the discovery that these organizations were luring people on Facebook. You could examine the statistics on people from all over the world going to join terrorist organizations and try to assess the impact of their "public relations" campaigns upon this phenomenon. You could also interview people, a small sampling, possibly, to ask about how their perceptions have been influenced by terrorist use of mass media in various modalities. You are likely to find that visual media have had a more powerful impact than words. The idea is to gather instances of terrorist efforts to inform and persuade, in multiple modalities, and then analyze their impact.
Alternatively, you may be investigating how various forms of mass media affect perceptions of terrorism, those driven by the media's choices of what to present or not present, as opposed to what terrorists have chosen to offer. An example of this distinction would be the choice of various television stations to not show the videos of beheadings, a mass media choice, as opposed to the terrorists' choice. Research into this issue involves finding instances of mass media's reporting on terrorism, perhaps doing a quantitative analysis of their frequency and duration, as well as a qualitative analysis, and then an analysis of their impact upon public perception. For example, you might look at a paper of national circulation, such as the New York Times to find examples of reporting on terrorism. How much space is devoted to reporting on terrorism on a given day? Is the reporting sensationalist or dry? Today, you can review the "comments" section of a news article, to see what the perceptions are. Similarly, a posting on Facebook can provide reporting and perception. Another avenue of exploration is to examine how the medium influences perception. Do more people feel called to action after viewing in a visual medium than do in a print medium? You will also note that in an election year, such as in the United State this particular year, that reporting on terrorism gets amplified by the candidates, who inevitably respond to the reporting to engage their constituencies. A discussion of that ripple effect could be the basis of an entire paper.
This is a very big issue to explore, no matter what approach you take, but an important issue, worthy of exploration. Remember that no matter what the thrust of the paper is, it is important to use examples to make the points you wish to make. Feel free to apply theories about perception, but always apply them to concrete instances of perception.