Remove the for loop and just do one pixel at a time.

Of course that means the final result is achieved in (600 x 409 = ) 245.400 frames instead of 1 frame. So you're gonna need some patience or find a way to change a variable number of pixels per frame. The code example below loads two images and then 'morphs' from one to the other via the pixel array. You can set the number of pixels that is changed per frame, from one to all the pixels in the image (or more). Currently it's set to two lines (500 x 2 = 1000 pixels) per frame.

Note: never put updatePixels() inside your for loop, because it's inefficient and will slow down your sketch.

Code Example

1. // this code assumes EQUAL DIMENSIONS for both images and sketch
2.  
3. int pixelsPerFrame = 1000; // set from 1 to 25.000
   
4. PImage bikeImg, carImg;
5. boolean toggleImage;
6. int count;
7.  
8. void setup() { 
9.   size(500, 500);
10.   background(255);
11.   bikeImg = loadImage("http://kamurangbike.com/image/cache/data/ScottMB/Scott_Spark_920_Bike-500x500-500x500.jpg");
12.   carImg = loadImage("http://www.refinedguy.com/wp-content/uploads/2012/09/sdiesel.jpg");
13.   image(carImg, 0, 0);
14. }
15.  
16. void draw() {
17.   loadPixels();
18.   for (int i=0; i<pixelsPerFrame; i++) {
19.     int mod = ++count % bikeImg.pixels.length;
20.     if (mod == 0) { toggleImage = !toggleImage; }
21.     pixels[mod] = toggleImage ? carImg.pixels[mod] : bikeImg.pixels[mod];
22.   }
23.   updatePixels();
24. }

I have seen this kind of coding befor since pixels is also an array you can copy to and from

Nice tip jane38!
Although I think he wants to see the "effect" happening gradually.  

// this code assumes EQUAL DIMENSIONS for both images and sketch

What about force a resize() on them?

  bikeImg = loadImage(bikePath + bikeFile);
  carImg  = loadImage(carPath + carFile);

  bikeImg.resize(width, height);
  carImg.resize(width, height);

Thanks for your quick replies to my question!
Using each loop for one pixel did it.
Now I can continue working on my idea for some image transforms.