Adobe Extending Flash Professional CS4 - Manual (Page 57)

35 EXTENDING FLASH CS4 PROFESSIONAL
BitmapInstance object
Example
The following code gets the height of the bitmap in pixels:
// Get the number of pixels in the vertical dimension.
var bmObj = fl.getDocumentDOM().selection[0];
var isBitmap = bmObj.instanceType;
if(isBitmap == "bitmap"){
var numVerticalPixels = bmObj.vPixels;
}
See also
bitmapInstance.hPixels