PImage s;

void setup()
{
  size(800, 800);

  s = loadImage("58282082_6123806342093_5453113237480407040_n.png.jpg");

  surface.setResizable(true);
}

void draw()
{
  background(255);
  imageMode(CENTER);
  float scale = min(width / s.width, height / s.height);
  float sw = scale * s.width;
  float sh = scale * s.height;

  image(s, width/2, height/2, sw, sh);
}