What the mask is supposed to do? Where this 198 value comes from? What are you trying to do exactly?

If you want to set some transparency to all the pixels of the image, it would be easier/faster to just set the alpha bytes directly in the pixel array of that image:
wrkImage.pixels[i] |= 198 << 24;

If you want to uniformly change the transparency of an image - aka set the transparency of all pixels to the same value - then you can also use tint(). In the example below you can use the mouseX to dynamically change the transparency. Far left is fully transparent, far right is fully opaque. Just move your mouse from left to right to see the effect (it may take a few seconds to load the images from the internet though).

Code Example

1. PImage [] imageArray = new PImage [3];
2.  
3. String url0 = "http://hortla.wsu.edu/images/LandscapeArchitecture.jpg";
4. String url1 = "http://photographystudioss.com/wp-content/uploads/2011/06/Landscape-Photos.jpg";
5. String url2 = "http://landscaping.savvy-cafe.com/wp-content/uploads/2007/03/irish-landscape.jpg";
6.  
7. void setup() {
8.   size(800, 800, P3D);
9.   imageArray[0] = loadImage(url0);
10.   imageArray[1] = loadImage(url1);
11.   imageArray[2] = loadImage(url2);
12.   noStroke();
13. }
14.  
15. void draw() {
16.   background(255);
17.   translate(width/2,height/2);
18.   tint(255,map(mouseX,0,width,0,255));
19.   for (int i=0; i<imageArray.length; i++) {
20.     int zMove = 500 - i * 250;
21.     beginShape();
22.     textureMode(NORMALIZED);
23.     texture(imageArray[i]);
24.     vertex(-imageArray[i].width/2, -imageArray[i].height/2, zMove, 0, 0);
25.     vertex(imageArray[i].width/2, -imageArray[i].height/2, zMove, 1, 0);
26.     vertex(imageArray[i].width/2, imageArray[i].height/2, zMove, 1, 1);
27.     vertex(-imageArray[i].width/2, imageArray[i].height/2, zMove, 0, 1);
28.     endShape();
29.   }
30. }

It's also possible to change the image transparency through the pixel array directly (which gives more control, for example to do non-uniform pixel manipulation). But in many cases the default PImage format seems to be RGB, rather than ARGB. So you may need to explicitly create an ARGB image or it won't work.

Code Example

1. PImage [] imageArray = new PImage [3];
2.  
3. String url0 = "http://hortla.wsu.edu/images/LandscapeArchitecture.jpg";
4. String url1 = "http://photographystudioss.com/wp-content/uploads/2011/06/Landscape-Photos.jpg";
5. String url2 = "http://landscaping.savvy-cafe.com/wp-content/uploads/2007/03/irish-landscape.jpg";
6.  
7. void setup() {
8.   size(800, 800, P3D);
9.   imageArray[0] = loadImage(url0);
10.   imageArray[1] = loadImage(url1);
11.   imageArray[2] = loadImage(url2);
12.   maskImageArray(125);
13.   noStroke();
14. }
15.  
16. void draw() {
17.   background(255);
18.   translate(width/2,height/2);
19.   for (int i=0; i<imageArray.length; i++) {
20.     int zMove = 500 - i * 250;
21.     beginShape();
22.     textureMode(NORMALIZED);
23.     texture(imageArray[i]);
24.     vertex(-imageArray[i].width/2, -imageArray[i].height/2, zMove, 0, 0);
25.     vertex(imageArray[i].width/2, -imageArray[i].height/2, zMove, 1, 0);
26.     vertex(imageArray[i].width/2, imageArray[i].height/2, zMove, 1, 1);
27.     vertex(-imageArray[i].width/2, imageArray[i].height/2, zMove, 0, 1);
28.     endShape();
29.   }
30. }
31.  
32. void maskImageArray(int transparency) {
33.   for (int i=0; i<imageArray.length; i++) {
34.     if (imageArray[i] != null) {
35.       PImage wrkImage = createImage(imageArray[i].width, imageArray[i].height, ARGB);
36.       for (int p=0; p<imageArray[i].pixels.length; p++) {
37.         color argb = imageArray[i].pixels[p];
38.         int r = (argb >> 16) & 0xFF;
39.         int g = (argb >> 8) & 0xFF;
40.         int b = argb & 0xFF;
41.         wrkImage.pixels[p] = (transparency << 24) | (r << 16) | (g << 8) | b;
42.       }
43.       imageArray[i] = wrkImage;
44.     }
45.   }
46. }