In the first code snippet, you can just call size before the loadImage() to the wanted size.
In the second code snippet, even if it is a common practice, it is to avoid: code before size() is generally called twice! That's why it is advised that size() should be the first call of setup().
The real problem of the second code is that you load the image in a local variable named left that disappears as soon as setup() is finished, while you still use image(img), hence the NullPointerException as it isn't initialized.

The right syntax should be:

I just tried, and apparently I don't get the "double run before size()" effect on this code snippet. On 1.5.1 and 2.0b7. So I am not sure when it can happen (but I saw it in the past).
Anyway, this works:

1. PImage left, right;
2.  
3. void setup() {
4. //  size(100, 100); // For example
5.   println("Loading images");
6.   left = loadImage("left.jpg");
7.   right = loadImage("right.jpg");
8.   size(left.width + right.width, left.height);
   
9. }
10.  
11. void draw() {
12.   image(left, 0, 0);
13.   image(right, left.width, 0);
14. }