The pivot point of a bitmap's rotation.

I want to change the orientation of a bitmap (square, 52x52 pixels). It works fine but my only problem is that I want the pivot point of the rotation to be the center of the bitmap. I'm not sure what is currently being used to do the rotation.

Any idea?

    function dessine_boussole(dc, l, xPos, myHeading, cTitre){

        var transform=new Graphics.AffineTransform();

        myHeading=360-myHeading;

        var radians=Math.toRadians(myHeading);

        transform.rotate(radians);

        dc.setAntiAlias(true);

        dc.drawBitmap2(xPos,l+60,

                        boussole,

                        {   :bitmapX=>2, :bitmapY=>2,

                            :bitmapWidth=>48, :bitmapHeight=>48,

                            :transform=>transform});