property bitmap

This property returns the bitmap resource currently loaded into the image box.
See the bitmap object for more details.

var imgBox = scene.getBoxById("startImgBox");
if (imgBox)
{
    var bitmap = imgBox.bitmap;
    if (bitmap)
    {
        shell.print("width = " + bitmap.width);
        shell.print("height = " + bitmap.height);
    }
}