OT- Neat, have you considered incorporating non-Wikimedia Commons images in your public domain-only search? Flickr in particular has a surprising amount of quality highres images that the authors have released into the PD.
Arguably, such images should be mirrored onto Commons ASAP (if they plausibly have educational value and are PD in most of the world). It is unwise to rely on a single profit-oriented company like Flickr to keep hosting this sort of stuff for the foreseeable future, whereas Commons is independently supported and specifically intended as a host for permissively-licensed media content.