Use this method.
/**
 * Rotates an image. Actually rotates a new copy of the image.
 * 
 * @param img The image to be rotated
 * @param angle The angle in degrees
 * @return The rotated image
 */
public static Image rotate(Image img, double angle)
{
    double sin = Math.abs(Math.sin(Math.toRadians(angle))),
           cos = Math.abs(Math.cos(Math.toRadians(angle)));
    int w = img.getWidth(null), h = img.getHeight(null);
    int neww = (int) Math.floor(w*cos + h*sin),
        newh = (int) Math.floor(h*cos + w*sin);
    BufferedImage bimg = toBufferedImage(getEmptyImage(neww, newh));
    Graphics2D g = bimg.createGraphics();
    g.translate((neww-w)/2, (newh-h)/2);
    g.rotate(Math.toRadians(angle), w/2, h/2);
    g.drawRenderedImage(toBufferedImage(img), null);
    g.dispose();
    return toImage(bimg);
}
taken from my ImageTool class.
Hope it helps.