AffineTransform mirrorY = new AffineTransform(-1, 0, 0, 1, 0, 0);
AffineTransform mirrorX = new AffineTransform(1, 0, 0, -1, 0, 0);
BufferedImage oldPic = ...;
BufferedImage newPic = new BufferedImage( oldPic.getWidth(), oldPic.getHeight(), oldPic.getType() );
for ( int x = 0; x < newPic.getWidth (); x++ )
for ( int y = 0; y < newPic.getHeight(); y++ )
picNew.setRGB( oldPic.getWidth() - x - 1, y, picOld.getRGB() );