Lo que necesitas se llama vector de movimiento y un poco de trigonometria basica.
Código ActionScript :
var centroX:Number = stage.stageWidth/2;
var centroY:Number = stage.stageHeight/2;
var vx:Number = 0;
var vy:Number = 0;
var imagen:DisplayObject = getChildAt(0);
addEventListener(Event.ENTER_FRAME,onEnterFrame);
function onEnterFrame(event:Event) : void
{
var rads:Number = Math.atan2(mouseY-centroY,mouseX-centroX);
rads += Math.PI;
vx = 15 * Math.cos(rads);
vy = 15 * Math.sin(rads);
var posX:Number = imagen.x + vx;
var posY:Number = imagen.y + vy;
posX = Math.min(0,posX);
posY = Math.min(0,posY);
posX = Math.max(-imagen.width+stage.stageWidth,posX);
posY = Math.max(-imagen.height + stage.stageHeight, posY);
imagen.x = posX;
imagen.y = posY;
}Saludos!!