Testing some queries against a copy of the database to find images w/o tags that might easily be recognized as deserving of a certain type of tag. This is against the January 7 dump, so it's out of date for meaningful work – most of these will already be tagged. E.g.:

"Public" but not tagged

edit

If the image description has the word "public", it might very well be described as public domain. So let's look for all untagged images with "public" in the description field:

select img_name, img_description from image where img_description like '%public%' and img_description not like '%{{%' ;

Name Description
Image:022srUSGSCyprusVia.jpg USGS public domain image. 22. Side view of support-column failure and collapsed upper deck, Cypress viaduct. [H.G. Wilshire, U.S. Geological Survey]
Image:0817tmp3.jpg 2004 Ford Territory. Ford New Zealand publicity photograph.
Image:122-38.gif USGS public domain faultline image
Image:1837flagsm.jpg Republic of Canada flag (1837)
Image:18th_century_bidet.jpg An etching from the 18th Century of ladies using bidets (now in the public domain).
Image:1905WhiteAuto.JPG Advertisement now in the public domain
Image:1946LincolnZepher.jpg 1946 Lincoln Zephyr, cropped from photo put in public domain on loc.gov
Image:1967-VW-Beatle.jpg Picture taken by User:Minesweeper on December 14, 2003 and released into the public domain.
Image:1973_C150L_19370.jpg Picture of (my) cessna 150L - public domain
Image:1_Norwegian_krone_adverse.jpg Taken from the official website of the Central Bank of Norway, which is owned by the Norwegian state, so public domain is assumed.
Image:1_Norwegian_krone_reverse.jpg Taken from the official website of the Central Bank of Norway, which is owned by the Norwegian state, so public domain is assumed.
Image:2002_Winter_Olympics_flame.jpg public domain
Image:2004GOPconventionlogo.png 2004 Republican National Convention logo
Image:2004_canadian_election_approval_ratings_June_2.png Approval ratings (percentages) for the 2004 Canadian federal election, created by Nicholas Moreau, public domain
Image:2MASS_logo.jpg 2MASS logo: images are public domain [1], c/o 2MASS/UMass/IPAC-Caltech/NASA/NSF
Image:4-42_FA_Coat_of_Arms.PNG This file is from a US Government website (http://www.hood.army.mil/4ID_4-42fa/coat_of_arms.htm) and so, to the best of my knowledge is in the public domain
Image:47b4d728b3127cceb66e1e3b27280000005610.jpg Picture found on Ebay, should be public domain
Image:50Cent.jpg publicity photo from official site, upped as fair use
Image:5p_pwy_pansies.jpg purple, white & yellow pansies. I release this picture into the public domain; you do not need my permission to reuse it, but do not claim that you took it.  :-) --KQ
Image:6-Genghis-Khan.gif This image is in public domain.

This could also be used to find images claimed to be PD, but probably are not. That EBay image, for example – I doubt that there's anything in the US law that says you lose your copyright by putting an image on an EBay page.