The Tachyon plugin is responsible for replacing WordPress' default thumbnail handling with dynamic Tachyon URLs.
Install the Tachyon plugin as a regular plugin in your WordPress install (mu-plugins also supported).
You also need to point the plugin to your Tachyon server. Add the following to your
define( 'TACHYON_URL', 'http://localhost:8080/<bucket name>/uploads' );
The Tachyon plugin is based on the Photon plugin code by Automattic, part of Jetpack. Used under the GPL.