1. A method for zooming an image on a touch screen having edges, the method comprising: detecting, by one or more processors, a trajectory on the touch screen formed by a continuous direct touching of the touch screen by a finger of a user, the trajectory beginning at a first point on the touch screen initially touched by the user and ending at a second point on the touch screen being currently touched by the user; determining, by the one or more processors, a direction given by a tangent to the trajectory at the second point, wherein the direction is measured with respect to mutually-orthogonal axes oriented parallel to the edges of the touch screen; and in response to determining that the angle is at least equal to a predetermined minimum angle relative to the mutually-orthogonal axes, initiating, by the one or more processors, either zooming-in or zooming-out of an initial image on the touch screen based on the trajectory.