Wikimedia Commons, Wikipedia's media database, has over 21 million images that are free and usable for commercial purposes:
http://commons.wikimedia.org/wiki/Category:Public_domain
Though I'm not sure how you can search just that subset.
Found it, add +incategory:"Public Domain" to the search, as in this example searching for "cats":
https://commons.wikimedia.org/w/index.php?title=Special%3ASe...
Example result (interestingly tagged as both public domain and CC):
"Two cats, dressed as humans, holding rope, which doll appears to be skipping"
https://commons.wikimedia.org/wiki/File:Two_cats,_dressed_as...
Though, from the dearth of results that may not be searching sub-categories.
The main issue with pointing to Wikimedia is that it seems to funnel people into Creative Commons licenses. Maybe that is not the intent, but that is what happens. I like the idea of a prominent place that encourages the Public Domain.
I also think a lot of people of the current generation probably don't think that much about licensing and copyright and don't realize how many strings they're attaching to the use of their image when they upload it, when my guess is that for most people, one of the CC licenses like CC-by-SA is essentially the same as releasing it into the public domain (for them), but of course it excludes it from inclusion into any non-CC works for likely over 100 years.