I'm setting up a gallery which slowly composites image on top of one another with a low opacity, so they layer over time.

I was wondering if there was a way to fade images in slowly rather than just have them appear suddenly? Sorry, I'm new at processing so any help would be appreciated.

My (awkward) code:

int maxImages = 10; // Total # of images
int imageIndex = 0; // Initial image to be displayed is the first
int fader = 25; //intializing the opacity start
float a = 10; //initiliazing opacity variable a
float aspeed = 2; //rate of change for a
PImage[] images = new PImage[maxImages]; // The image array

Timer timer;
void setup() {
size(600,600);
// Loading the images into the array
for (int i = 0; i < images.length; i ++ ) {
images[i] = loadImage( "animal" + i + ".jpg");
//starts the count
timer = new Timer(2000);
timer.start();
}
}


void draw() {
if (timer.isFinished()) {
tint (255, a);
image(images[imageIndex],0,0);
// increment image index by one each cycle
// use modulo "%" to return to 0 once the size
//of the array is reached
imageIndex = (imageIndex + 1) % images.length;

timer.start();
//changes a over time
a = a + aspeed;
// Remember, || means "or."
if ((a > 22) || (a < 10)) {
// If a goes over 22, multiply speed by -1 to turn it around.
aspeed = aspeed * -1;
 }
}
}

class Timer {
int savedTime; // When Timer started
int totalTime; // How long Timer should last
Timer(int tempTotalTime) {
totalTime = tempTotalTime;
}
// Starting the timer
void start() {
savedTime = millis();
}
boolean isFinished() {
// Check how much time has passed
int passedTime = millis()- savedTime;
if (passedTime > totalTime) {
return true;
} else {
return false;
}
}
}