User blog comment:DaNASCAT/Technical Update: May 6, 2014/@comment-452-20140507210548

Alternative method to hide username below thumbnails:
 * 1) Create MediaWiki:Thumbnails-added-by with &amp;#32; (Character 32 is a normal space, FYI)
 * 2) Purge pages with thumbnails to see the change.

This has the downside of not affecting cached pages, but the upside of the username being removed from the page source entirely.