There are many utilities available to do this tiling with pictures. I did a quick search using "mosaic pictures software" and got hundreds of hits. Try soft411.com/software/mosaic-pictures.html Utility costs $12 US.
So after checking out the link above, as well as doing my own quick search, I have learned a little more about this art. In a real photo-mosaic, the colour tone of the micro-images is adjusted so that it closely matches the colour of the primary image.
I used a much simpler technique when I did my Photoshop experiment. First of all, I used a small number of "micro-images" because I wanted to experiment with the technique rather than get bogged down with the sheer volume of manipulating some 100 images. I used eight images as my micro-images, arranged in two rows of four. Then I duplicated the two rows to create the four by four matrix. I performed a mirror transformation on the duplicate rows so it would not be too obvious that there was a duplication.
I brought the primary image into a separate layer that I positioned on top of the micro-image layer. I made the primary image layer partially transparent in order to allow the micro-images to come through. By doing so, I didn't have to adjust the colour tone of my micro-images.
Now, if I was going to apply this technique on a larger scale, say using a underlying grid of 10 by 10, or 25 by 25, it would take a lot more work on my part. However, I know that Photoshop has features for automating repeated tasks, so I could use this as a learning exercise to learn how to use those features.