var bitmapData_1:BitmapData=new BitmapData(MovieClip(root).width,MovieClip(root).height); // generate bitmapData with root size
image.draw(MovieClip(root)); // drawing the BitmapData (root image)
var posX:uint = posY:uint  = 0;
var recWidth:uint = recHeight:uint = 100;
var bitmapData_2:BitmapData=new BitmapData(recWidth, recHeight); // generate second BitmapData with the width/height we want
bitmapData_2.copyPixels(bitmapData_1, new Rectangle(posX, posY, recWidth, recHeight), new Point(0,0)); // put the image into the second BitmapData and draw from 0,0 top left