I used RipMe to grab about 3800 images from the sub. About 1GB.
AndreaMosaic was useed to generate the photomosaic.
The image posted to Reddit drew from all of the downloaded images, but its layout used fewer (~3700). I believe repeats were capped at.. 10?. What this means is: it doesn't depict every single HCA post. Using every image as a unique tile means the set of tiles is mostly white or black walls of text, with faces probably being a predominant picture element, and colors drawn from a pretty small random pool.
Ideally this would be a live thing in the cloud that updates and regenerates weekly, or on-demand over a range of dates, etc. yadda yadda, knobs-to-twiddle.
The winner in the middle was featured in this post.