int angle; //the rotation variable
PImage img; 

void setup(){
  size(200, 200);
  imageMode(CENTER);
  img = loadImage("YOUR_IMAGE");
  angle = 0;
}

void draw(){
  background(255);
  pushMatrix(); //push Matrix so we can rotate
  translate(width/2, height/2); //translate to where you want it
  rotate(radians(angle)); //rotate the image 
  image(img, 0, 0); //draw at 0, 0 since we translated
  popMatrix(); //don't forget to pop the matrix
}

void keyPressed(){
  if(keyCode == 37){ //arrows are coded so you can't call "key"
    rot--; //left arrow rotates one way
  } else if(keyCode == 39){ 
    rot++; //right arrow rotates the other way
  }
}