I’ve been searching for public domain clip arts for the previous week or so more for research purposes, just to see what are going on and who’s doing what. I stumbled across this article pointing to a library called wpclipart and it looks like the best raster public domain library I’ve found thus far. There’s lots of other sources of public domain images like the NOAA photo library , which actually is one of the libraries that I like to browse through some times. The problem with NOAA library is categorization. It was not meant to be a clip art library and that’s why it is hard to find what you’re looking for there, unless you really know where it is or the search is very obvious. Some images do not have descriptions or comments.
However, the NOAA photo library is a great source for those who want to trace image and create vector cliparts, or combine parts of those different images together as there is no copyrights for any of the images.