This animates a sequence of images without an animated gif library. Basically stealing from the sequential example on processing.org

Animation without gifs

/* Animated with no library demo
Steve Lambert April 26, 2017
*/

// https://processing.org/examples/sequential.html

int numFrames = 4;  // The number of frames in the animation
int currentFrame = 0;
PImage[] images = new PImage[numFrames];
    
void setup() {
  size(250, 250);
  frameRate(24);
  imageMode(CENTER);
  
  // you can load things this way
  
  //images[0]  = loadImage("red-0000.png");
  //images[1]  = loadImage("red-0001.png"); 
  //images[2]  = loadImage("red-0002.png");
  //images[3]  = loadImage("red-0003.png"); 
  
  // or you can load things more cleverly like this:

  for (int i = 0; i < numFrames; i++) {
    String imageName = "red-" + nf(i, 4) + ".png"; // nf converts #s to strings
    images[i] = loadImage(imageName);
  }
  
} 
 
void draw() { 
  background(0);

  if (frameCount % 20 == 0) { // this modulo slows it down to every 20th frame 
  
    currentFrame = (currentFrame+1) % numFrames;  // Use % to cycle through frames
    }
    
  image(images[(currentFrame)], width/2, height/2); // display the current frame
}