NOAO Image Gallery http://www.noao.edu/image_gallery/
Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html A new astronomy-related image every day. Plus archives of all previous images.
LPI Slide sets http://www.lpi.usra.edu/publications/slidesets/lpislides.html These can be ordered as slides for a fee or downloaded as .gif files for computer use.
Multiwavelength Milky Way http://nvo.gsfc.nasa.gov/mw/milkyway.html Images of our galaxy in different wavelengths (colors).
PlanetScapes http://planetscapes.com/ Views of the Solar System - lots of images with descriptions.
NASA Planetary Photojournal http://photojournal.jpl.nasa.gov/ Almost every image ever taken by a NASA spacecraft can be found here.
NASA- JSC Digital Image Collection http://images.jsc.nasa.gov/ Images taken from shuttle missions and press release images from the manned space program.