You asked about Charis Tsevis's work. Charis uses Studio Artist to generate his photo mosaic work.

Here are some articles on his Studio Artist work.

He uses tiling movie brushes to generate the photo mosaic effects.

Here's a tip on creating your own movie brush to make a photo mosaic image.

Here are a large number of blog articles on different photo mosaic and photo collage effects.

Photo mosaic effects and how to duplicate Charis's work has been extensively discussed here on the users forum over the years. So you should do some searching through the archived posts here. Here's one old post to take a look at.

Here's a post that very specificly describes an approach you can use to create sub-nested tiling photo mosiacs, whihc is somethign that Charis does using Studio Artist quite a bit.