As far as I know all of these webapges are sources of free astronomical images. It is possible that some of them still have copyrights to their images, so please be sure to check that before using them.

• Anglo-Australian Observatory - lots of astronomy images and other astronomy info
• Astronomical Image Library - searchable index for astronomy pictures on the web (website is down as of July 20, 2011 with no date for when it will return)
• Astronomy Picture of the Day - a picture and description (written by a professional astronomer) that changes every day
• Canadian Astronomy Data Centre (CADC) - access to many astronomical archives, surveys, and catalogs
• Earth Observatory - images of the Earth and events on the Earth from space
• Chandra X-ray Observatory Images
• Digitized Sky Survey - Find info on any object in the sky, including coordinates and pictures
• High Energy Astrophysics Mission Images - Links to images from high energy (X-ray, gamma ray, etc.) satellite observatories including: ROSAT, Einstein (HEAO-2), Copernicus (OAO-3), and many others
• Hubble Site Gallery - Hubble Space Telescope picture gallery
• Lunar and Planetary Observation and CCD imaging - amazing pictures and animations of the moon, sun, planets, stars...
• Lunar Photo of the Day - photo with description of the moon, changes daily
• Moonglow Observatory - small astronomical observatory run by Fred Bruenjes as a hobby, contains lots of pictures
• Multiwavelength Milky Way - images of the Milky Way in a variety of wavelengths (radio, infrared, optical, x-ray, gamma ray, etc.)
• NASA Multimedia Gallery - a large collection of NASA photos and videos of astronomy, rocketry, space missions, etc...
• NASA's Planetary Photojournal - contains over 800 publicly released images from various Solar System exploration programs
• National Optical Astronomy Observatory (NOAO) Image Gallery
• Observational Mishaps - image database of astronomical observations which exhibit problems one might encounter during observing runs
• SDSS Image Gallery - This page leads to collections of photos and other images associated with the Sloan Digital Sky Survey.
• SEDS Gallery - an archive of space images, pictures, art...
• Sky View Virtual Telescope - view images of celestial objects in a variety of different wavelengths (visual, radio, x-ray, gamma ray, ect...)
• Space Audio - sounds from space (very cool!)
• Stellar Scenes - excellent amateur astrophotography out of Japan (English and Japanese Versions)

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