Basically, its main function is to be displayed on the front of the room during the committee meeting, so that people can have a visual reference/cue of the paper being talked about.
Even though the representative image does not have any practical value for the paper (it is not illegal to use an arbitrary image, like, say, picture of Lindsay Lohan, at least with clothes on), the committee members are humans and thus can be subtly influenced by all psychological factors.
Thus, it is to your advantage to use an image that can help your paper, i.e. conveys the main ideas or results while looks appealing.