Digitized Sky Survey
Digitized images of the whole sky. Check out the other sites they list if you want lower-resolution images that don't take so long to load, or images of larger regions of the sky.
archive.stsci.edu Details

Earth and Moon Viewer
View the earth and moon from many vantage points and in multiple ways.
www.fourmilab.ch Details

Galactic Images
Astronomical pictures for sale.
www.galacticimages.com Details

Galaxies In Chaos
A variety of astronomy images. Including galaxies, stars, and nebula.
visitastronomy.com Details

Galaxy Cluster Mug Shots
From the U. of Alabama.
www.astr.ua.edu Details

Hubble Heritage Project
Team of astronomers and astrophysicists preparing images from Hubble Space Telescope.
heritage.stsci.edu Details

Hubble Heritage Project: Archive Gallery Page
More Hubble images.
heritage.stsci.edu Details

HubbleSite - Gallery
At the official online home of NASA's Hubble Space Telescope exists a gallery of the best pictures that the Hubble has taken.
hubble.stsci.edu Details

IPAC Gallery
Images and information.
coolcosmos.ipac.caltech.edu Details