Where can I find public domain astronomical pictures? Where can I find public domain astronomical pictures of nebulae, stars, etc. that can be freely used?
 A: If you want to search for images and see them as a layer against the sky (that is, in context), WikiSky.org is a great, community supported site.
If you want to search through a database of observations have a look at Las Cumbres Observatory's observations.
This is a public archive of automatically (that is, by a computer pipeline, not a person) processed astronomical images, but you can download the real data files and have a go at your own processing :) 
A: Typically you could try images at NASA which are mostly (if not all) public domain. You could start your exploration at: 
http://www.nasa.gov/multimedia/imagegallery/index.html
At NASA they explicitly mention that their images are public domain so you can use them as you wish. Similarly you could try some other big astronomical institutions around the world where they might also have those "copy waved" images.
A: As one more option, all the pictures on Wikimedia Commons are freely licensed or in the public domain.  In particular, you're free to distribute, modify and/or even sell them, although you may be required to credit the author(s) and possibly to release any derivative works under the same license.  They have plenty of high-quality astronomy pictures, both from NASA and from other sources.
You can start with their featured astronomy pictures or the astronomy category, or just search for the type of images you want.  Or you can just click on the images in Wikipedia; just about all of their astronomy images are free and hosted on Commons.

Another very useful starting point is the Creative Commons search tool, which lets you search specifically for freely licensed content from a number of sources, including Google, Flickr and Wikimedia Commons.
A: NASA still images; audio files; video; and computer files used in the rendition of 3-dimensional models, such as texture maps and polygon data in any format, generally are not copyrighted. You may use NASA imagery, video, audio, and data files used for the rendition of 3-dimensional models for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages. This general permission extends to personal Web pages.
http://www.nasa.gov/audience/formedia/features/MP_Photo_Guidelines.html
NASA should be acknowledged as the source of the material except in cases of advertising. See NASA Advertising Guidelines.
http://www.nasa.gov/audience/formedia/features/Advertising_Guidelines.html
A: Any NASA image is in the public domain, although it's common practice to provide attribution back to NASA.  (I often see people trying to make copyright claims on images that I know are NASA provided images)  I believe that other images provided by US government funded observatories are also public domain, but you occassionally get a PI institution trying to assert a claim.
The issue is going to be how you want to look for the images.  The two main places that I know, without going to a specific mission archive are:


*

*NASA's Astronomy Picture of the Day (APOD)

*NASA Image Exchange
However, these are often calibrated and cleaned up in ways that make 'em useless for scientific analysis.  If you want the really good stuff, you'll want to go to use the National Virtual Observatory (NVO), but then you'll need a FITS reader, and have to colorize the images yourself to make 'em pretty.
update : a few more sources of NASA images:


*

*NASA Images, a collaboration between the Internet Archive and NASA

*Google Directory : Science > Technology > Space > NASA > Images and Movies
