I am relatively new to Processing. I am attempting to overlay an image1 over an image2, treating white as transparent. I would like it to be done efficiently.

I thought that the"mask()" function would do this but could not get the example to work. I also tried someone's example which loaded the top image and set the white pixels fully transparent. I could not get even his example to work.

Ideas would be appreciated.

Does white have to be the transparent color?
if not, create a PNG in photoshop, leave the background transparent and save it... just use this png as img and youve got all the transparency you need. I would say thats the best way to do that.

you could try using the method MULTIPLY of the blend() function, that makes white transparent.

check the reference session of processing's site and look for it.

you should use this kind of statement blend(img, 0, 0, width, height, 0, 0, width, height, MULTIPLY); and place it after image() is called [or after you update the pixels]. on this example 'img' is the top image you want to merge with a bottom layer.

well, there is another idea to make only the white transparent.

you can read the pixels[] of the image you want to have the transparency looking for white pixels [or defining a threshold for pixels lighter than x]. you can read the array using a for iteration and at the same time use a boolean[] defining false for pixels that should be opaque, for example, and true for the ones that should be transparent [use if statement comparing value of pixel with threshold if statement]. this boolean array should have the same length of the image's pixels array [image width * image height].

after analyzing each individual pixel and defining which ones should be transparent you can print the background image to the screen with image() and output the upper layer [the one with transparent whites] using the pixels[] and an if structure based on the boolean array that states which frames should and which should not be transparent (for this you should remember to call loadPixels() before working with the pixels[] and updatePixels() to update them on the screen ).

pedro veneroso, Dusan Licer, I suggested something like that in my first answer (although in a very terse way), but I suggested to alter the pixels right away, without using a secondary array. No need for mask, then.

Not sure if the OP is still interested by the topic...

You are right.
I checked the source, and indeed the problem is when we load an image without transparency, the image is in RGB format, without opacity info.
I found out that forcing the format to be ARGB works...
Code:

size(500, 500);
PImage img = loadImage("E:/Documents/Images/References/parelss3.jpg");
stroke(#0055FF);
strokeWeight(4);
for (int x = 0; x < width; x += 20)
  line(x, 0, x, height);

image(img, 10, 10, 200, 200);
// Not necessary for a PImage
//    img.loadPixels();
for (int i = 0; i < img.width * img.height; i++)
{
  if ((img.pixels[i] & 0x00FFFFFF) == 0x00FFFFFF)
  {
    img.pixels[i] = 0;
  }
}
img.format = ARGB;
img.updatePixels();
image(img, 200, 200, 200, 200);