PImage pic;
PImage pic_flipped; // keep a cached version in here

boolean drawFlipped = false;

void setup()
{
    pic = loadImage( "Picture.png" );   
}

void draw ()
{
    background(123);
    
    if ( drawFlipped )
	  image( flipPic(pic), 10, 10 );
    else
	  image( pic, 10, 10 );
}

void mousePressed ()
{
    drawFlipped = !drawFlipped;   
}

PImage flipPic ( PImage _img )
{
    if ( pic_flipped != null )
	  return pic_flipped;  // cached version
    else
	  pic_flipped = new PImage( _img.width, _img.height, _img.format );
    
    // copy pixels over ..
    int indx = _img.width * _img.height;
    int indxF = 0;
    for ( int y = _img.height; y > 0 ; y-- )
    {
	  indx -= _img.width;
	  System.arraycopy( _img.pixels, indx, 
						 pic_flipped.pixels, indxF, 
						 _img.width );
	  indxF += _img.width;
    }
    
    return pic_flipped;
}