Make two arrays, the first set would be your "image one" set, the second would be your "image two" set. Make sure the pairs sync up across the two sets (the first in one matches the first in the second and so on). Then all you should need to do is pick one random number and use it to select from both arrays and you should be all set.

Some JS towards that end:

// make two arrays
var img_one = new Array();
var img_two = new Array();

// populate the two arrays so that the contents sync up.
img_one[0] = 'leftone.jpg';
img_two[0] = 'rightone.jpg';
img_one[1] = 'lefttwo.jpg';
img_two[1] = 'righttwo.jpg';

// generate a random number limited by the number of options available.
var n=Math.floor(Math.random() * img_one.length);

// write out the left image.
document.write('<img src="'+img_one[n]+'" alt="left image" />');

// write out the right image.
document.write('<img src="'+img_two[n]+'" alt="right image" />');

But the same basic idea applies to however you decide to do this, PHP, JS, whatever.

PHP would be able to automate the process by reading the contents of a directory or something. But in the end it sorta breaks down to that "ask 6 people get 6 different answers" kind of situation.

Plus, the thread was in the JS category. ;)