This is the code that powers https://placem.at. Unless you want to submit changes to the application itself, you probably don't need to worry too much about this. You can just head over to the Placemat site and start loading in some images!
If you do want to submit changes, the full list of source images is in db/seeds.rb. There's also an example imgix config at config/imgix.yml.example, and an S3 config at config/s3.yml.example. That should theoretically be all you need to know to get up and running. The seeds are idempotent, so feel free to run rake db:seed whenever they're changed.
MIT-licensed, contributions welcome. Just fork and follow standard GithHub Flow.