int imgcount = 2;
String fontname = "font.vlw";
int imgnum = 0;
float startx, starty;
int transp;

void setup(){
  size(640,480);
  background(0);
}

void draw(){
 
  loop();
  background(0);
  PImage img = loadImage(imgnum+".jpg");
  startx = 640 - img.width;
  starty = 420 - img.height;
  
  tint(255, transp);
  image(img, startx/2, starty/2, img.width, img.height);  
  
  PFont fontA = loadFont(fontname);
  textFont(fontA, 12);
  fill(255);
  imgnum++;
  imgcount++;
  text("Image "+imgnum+" / "+imgcount+", use the left and right arrows to navigate.", 160, 450);
  imgcount--;
  imgnum--;
}

void keyPressed(){
  if (keyCode == RIGHT && imgnum < imgcount) { imgnum++; transp=0; }
  if (keyCode == LEFT && imgnum > 0) { imgnum--; transp=0; }
  redraw();   
}

void loop() {
  if ( transp < 255 ) { transp = transp + 20; }
}