I have been asked rather often where I found many of the pictures shown in my lectures. The following is a (rather incomprehensive) list of good starting places:

• NASA Planetary Photojournal: pictures of all the planets, high quality, even going back to the early ages of planetary exploration.
• Astronaut photography of the Earth: the largest online collection of astronaut photography of the Earth
• Press Releases from the European Southern Observatory often contain stunning pictures of extrasolar objects
• The Space Telescope Science Center focuses on pictures from the Hubble Space Telescope. The text of the releases is sometimes too hyped up, but the pictures are of outstanding quality.
• SEDS (Students for the Exploration and Development of Space) is a non-profit organisation with a good collection of pictures.
• Astronomy Picture of the Day has a new astronomy picture every day, with good explanations; all fields of astronomy are represented here, and a very good index is also available.
• The image gallery of the National Optical Astronomical Observatory in Tucson, AZ, is well sorted, has a good galaxy collection.
• Pictures taken with the Isaac Newton Group of telescopes on La Palma.
• allthesky.com also has a rather nice collection of digital images.
• For artistic illustrations, www.spaceart.org is a good starting point to many individual artists.