Lib-linkaged-BitmapData ignores the width and height?

Well, I have a bitmap imported to my lib, and I linkaged it with a name “PNGBitmapData”,
Then I can use new PNGBitmapData(WW,HH) to create this bitmap, it’s very convenient. But as the constructor of “BitmapData” receives two parameters as width and height, i can specify the size of this PNG image.
But no matter what number i use, even (0,0), the bitmap is just shown there. Nothing is different.
So, are these numbers just ignored by the FlashPlayer ?