This is a little utility workflow that allows you to shrink your PNG files. The workflow uses the service provided by http://tinypng.org, which also explains what exactly happens when PNG files are shrunk. You need an active internet connection for this to work.

Included in this workflow are File Actions (action: Shrink with TinyPNG), File Filters (keyword: tinypng) and a configurable Shortcut. Once you select a PNG file / or files and invoke the workflow, it will create a backup of your original file with the prefix "original_" and then compress your file using the web service.