For random images, here's the general idea using Build List (use your own folder of images with the List Files node). I put this together quickly, so there may be some room to optimize a bit. The gray nodes pictured were added to the previous comp.
See attached. I approached this by setting up the XY list of points first, then copying that n times for the Z positions.
You'll see I was also playing around with lights, because only using the lit image shader, images on some 3D grid points didn't show up so well. As a result I have a separate issue -- is there a way to get the camera to process it's position before the light? Using a spot light clearly illustrates the issue -- when starting up the comp, the spot light shows its position as expected, then as soon as the camera is moved, the points keep the light position from the first frame.