he simplest way to add some vitality to a composition is to use a photo or a drawing of a naturally dynamic object with an expressive motion theme, such as a car, a running businessman, or a flying bird. Of course such an image is likely to turn into something more important than just a dynamic add-on, so you'll have to exercise your entire inventory of design approaches (such as the analysis of colors, shapes, sizes, textures, etc.) in order to find a good way of inscribing it into your composition. Here we'll only focus on those requirements and limitations that result from the dynamic nature of such an element.

First off, to be able to move without hindrance, an object needs some empty space around it. That's why an element with a strong motion implication may require rearranging the neighboring elements so as to give it some more elbow room, otherwise its motion will "bump into a wall" without producing much of a dynamic effect - or even causing an undesirable impression of an obstacle or entanglement.

If there's more empty space on the trailing side of a dynamic object than in front of it, then the movement is perceived as decelerating, about to stop. This is sometimes accompanied by bending the vector of motion downwards, as we're accustomed to seeing a downwards movement as a final stage of a flying body's trajectory.

On the other hand, if the object has more room to move ahead than behind it, or if its "nose" is above its "tail," we perceive its motion as accelerating and "uphill." Interestingly, when the object is positioned horizontally and symmetrically with equal intervals on both sides, the perceived motion is not only of constant velocity but, in most cases, pretty much slowed down and unpersuasive. This is because we view asymmetry and acceleration as necessary attributes of any psychologically significant motion. Constant linear motion and the state of rest are equivalent not only in physics, but also in design (compare the visual equivalence of straight lines and circular arcs, both having zero curvature range).

Any motion introduces a vector, i.e. a line or direction which, as any other line in your composition, must be carefully coordinated with other dominant directions on the page. Of course in the case of complex, non-geometric images (such as a running figure), this line can be determined only vaguely, and it therefore naturally tends to "snap" to horizontals and verticals. To make perception more seamless, I'd recommend emphasizing the supposed direction of motion by a parallel line positioned close to the dynamic object.

On the other hand, the imprecise yet powerful direction defined by a realistically dynamic image could be used to add a diagonal theme to the composition in an obvious yet unobtrusive manner. As I once mentioned, horizontals and verticals are dominant in almost any composition if only because of the rectangular nature of computer screen and window borders, so breaking this directional monotony in a persuasive way may be difficult. Dynamic images, with their implicit direction without an explicit line, may be very useful here.

Let's see an example. On the Kasparov vs Deep Blue site (that I already referred to before), the expressively dynamic athlete photo defines a diagonal descending direction (top left to bottom right), which is additionally emphasized by the background arc of the earth's horizon seen from space. Both elements avoid being too precise in this directional innovation: both the arc and the athlete's arms and legs are not straight lines, and their implied directions are close but not parallel, producing a fuzzy bunch of diagonals rather than a single definite direction.