Also the camera feed flickers constantly

It may help to use a buffer.

I added the buffer but still getting the same flickering issue. Also the video is not supposed to start until the value within the sensor range is met but the camera is always on.

here is my processing code with the buffer and another tweak.
int imgCount= 12;
PImage [] imgs = new PImage[imgCount];
float imgW;
float offset = 0;
float easing = 0.05; //using sensor value distance to change its position. this program overlays one image over another by modifying the alpha value of the image with the tint() function.

int currentImage=0;
int lastShown=0;
int interval=3000; // each slide shows for this many millis
int blendedInterval=9000;

int lastRangeReading = 0;
int videoResetInterval=1000; // if we haven't gotten any close range values for a second, reset video transparency

//keep track of loaded images (true or false)
boolean[] loadStates = new boolean [imgCount];

// for loading images
float loaderX, loaderY, theta;

import processing.video.*;
import processing.serial.*;


Capture video;
int count;
int writeRow;
int maxRows;
int topRow;
int buffer[];

boolean showLiveVideo=false;
Serial serialPort;
int rangeVal=255;

PImage videoBuffer;

void setup() {
  size (480, 640, P3D);
  background(0);
  smooth();
  imgW= width/imgCount;
  // load images asynchronously
  for (int i = 0; i < imgCount; i++) {
    imgs[i] = requestImage("warhol_img"+nf(i, 4)+".jpg");
  }

  // Setup VIDEO Capture

  String[] cameras = Capture.list();

  //video = new Capture(this, 640, 480);
  video = new Capture(this, cameras[9]);

  video.start();

  maxRows = height *2;
  buffer = new int[width * maxRows];
  writeRow = height - 1;
  topRow = 0;

  frameRate(10);
  background(0);
  loadPixels();

  // Setup SERIAL PORT
  println(Serial.list());

  String portName = Serial.list()[0];
  serialPort = new Serial(this, portName, 9600);
}

void draw()
{
  if (video.available()) {
    video.read();
    videoBuffer=video;
  }
  if (videoBuffer !=null) {
    image(videoBuffer, 0, 0);
  }
  // check for new range finder data
  while ( serialPort.available () > 0) {  // If data is available,
    rangeVal = serialPort.read();         // read it and store it in val
       lastRangeReading=millis();
    //println(rangeVal);
  }
if(millis()-lastRangeReading >videoResetInterval) {
    lastRangeReading =millis();
    rangeVal=255; // make video completely transparent   
 }


  // draw pollock image
  background(0);
  //tint(255, 255);
  //image(img, 0, 0, displayWidth, displayHeight);
  for (int i = 0; i < imgs.length; i++) {
    // Check if individual images are fully loaded
    if ((imgs[i].width != 0) && (imgs[i].width != -1)) {
      // As images are loaded set true in boolean array
      loadStates[i] = true;
    }
  }
  if (checkLoadStates()) {
    // all the images are loaded
    if (currentImage<imgCount) {
      // we are in the first set of single images
      drawNextImage();
      if (millis()-lastShown>interval) {
        currentImage=currentImage+1;
        lastShown=millis();
      }
    }
    else {
      // we are drawing the blendef image
      drawBlendedImages();

      if (millis()-lastShown>blendedInterval) {
        // if we've shown the blended image for long enough
        // reset the whole system
        currentImage=0; // start at first image;
        lastShown=millis(); // reset the timer
        tint(255);
      }
    }
  }
  // draw video image
  tint(255, rangeVal);  // display at half opacity
  image(video, 0, 0, 480, 640);
}

void drawNextImage() {
  int y = (height - imgs[0].height) / 2;
  image(imgs[currentImage], 0, y, imgs[currentImage].height, imgs[currentImage].height);
}

void drawBlendedImages() {
  int y = (height - imgs[0].height) / 2;
  for (int i = 0; i < imgs.length; i++) {
    //image(imgs[i], width/imgs.length*i, y, imgs[i].height, imgs[i].height);
    tint(255, 128);
    image(imgs[i], 0, y, imgs[i].height, imgs[i].height);
  }
}

// Loading animation
void runLoaderAni() {
  // Only run when images are loading
  if (!checkLoadStates()) {
    ellipse(loaderX, loaderY, 10, 10);
    loaderX += 2;
    loaderY = height/2 + sin(theta) * (height/8);
    theta += PI/22;
    // Reposition ellipse if it goes off the screen
    if (loaderX > width + 5) {
      loaderX = -5;
    }
  }
}

// Return true when all images are loaded - no false values left in array
boolean checkLoadStates() {
  for (int i = 0; i < imgs.length; i++) {
    if (loadStates[i] == false) {
      return false;
    }
  }
  return true;
}

void captureEvent(Capture c) {



  c.read();
}


here is my arduino code
// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  int sensorValue = analogRead(A0);
  // print out the value you read:
  //Serial.println(sensorValue);
  //Serial.println(map(sensorValue, 0, 545, 0, 255));
  Serial.write(map(sensorValue, 20, 225, 0, 255));
 
  // send sensor value as number between 0 - 255
  //Serial.write(sensorValue/4);
  delay(50);        // delay in between reads for stability
}

Maybe just uncomment parts of your code in draw() to see where the problem is.