Certainly one can influence one's own dreams--especially with practice.
However, a long list of people historic and present are living testimonies that SOME dreams are indeed messages from God.
Joseph in the Old Testament comes to mind along with Daniel.
Yeah, the thing is that Alqaida is relying on dreams for communication, which is kind of weird when prophets do this and supposedly the last prophet died over 1000 years ago and there will never be another according to them. Somebody should call them on that.
Not only that, it's relatively easy to influence OTHER people's dreams.