For astronomical images, you can’t do better than the "Astronomy Picture of the Day" website, http://antwrp.gsfc.nasa.gov/apod/astropix.html. Not only does this have a fabulous archive of the most amazing pictures (and a new one every day), each image also has links to many other interesting sites where you can follow up the topic. I’ve used APOD as the source for many of the images here, mostly because it’s so convenient

NASA has a site called "Imagine the Universe", http://imagine.gsfc.nasa.gov/index.html which is a good place to start for finding out about all things astronomical. It also has excellent links to other places on the Web to find information.

"Imagine the Universe" has a nice page about the electromagnetic spectrum: http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

For more galaxies and other objects in many wavelengths, take a look at IPAC’s "The Multiwavelength Astronomy Gallery", http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_museum/; or the "Multiwavelength Milky Way", http://mwmw.gsfc.nasa.gov/

Mike Brown, the discoverer of Eris, has an excellent blog about scale in the Solar System, and how hard it is to depict planetary scales accurately, at 
http://www.mikebrownsplanets.com/2009/08/planetary-placemats.html

There’s a lovely book on the same topic: "Sizing up the Universe: The cosmos in perspective" by J. Richard Gott and Robert J. Vanderbei (National Geographic, 2011). It’s like a meditation about size and scale, and the centrepiece is their "Logarithmic Map of the Universe" – a version of which can be found at 
http://www.astro.princeton.edu/universe/ (though not as pretty). xkcd has a similar idea in the cartoon just called "Height" http://xkcd.com/482/

The Galaxy Zoo project is a "citizen science" project where members of the public assist in research by classifying galaxies observed in the Sloan Digital Sky Survey: 
http://www.galaxyzoo.org/. There are other associated projects: classifying features on the Moon (https://www.zooniverse.org/project/moonzoo), finding planets around other stars (https://www.zooniverse.org/project/planethunters), and classifying Hubble galaxy pictures (https://www.zooniverse.org/project/hubble)

Hanny van Arkel has her own blog, where she talks about the discovery of her voorwerp: http://www.hannysvoorwerp.com/

Title image: The barred spiral galaxy NGC 613, taken by ESO's Very Large Telescope. From ESO Press Release, 19 December 2003 http://www.eso.org/public/outreach/press-rel/pr-2003/phot-33-03.html

Earth: View of the Earth from space, Eastern hemisphere, from "The Blue Marble: True-color global imagery at 1 km resolution" http://earthobservatory.nasa.gov/Newsroom/BlueMarble

Valles Marineris hemisphere of Mars: from NASA Planetary Photojournal, http://photojournal.jpl.nasa.gov/catalog/PIA00407

Cassini view of Jupiter: from NASA Planetary Photojournal, http://photojournal.jpl.nasa.gov/catalog/PIA02873

Asteroid Ida and its moon Dactyl: from Views of the Solar System by Calvin J. Hamilton http://www.solarviews.com/cap/ast/idamnclr.htm

Callisto (second largest moon of Jupiter): from NASA Planetary Photojournal, http://photojournal.jpl.nasa.gov/catalog/PIA03456

Comet: Comet Hale-Bopp in 1997, from APOD 2000 December 27 http://antwrp.gsfc.nasa.gov/apod/ap001227.html

Sun: APOD 2003 July 29 http://antwrp.gsfc.nasa.gov/apod/ap030729.html

Eagle nebula: APOD 1997 January 19 http://antwrp.gsfc.nasa.gov/apod/ap970119.html

M17: The Omega Nebula in Sagittarius APOD 2003 April 25 http://antwrp.gsfc.nasa.gov/apod/ap030425.html

Keyhole Nebula: Hubble Heritage Project, http://heritage.stsci.edu/2000/06/index.html

NGC 281: Star formation around the open cluster IC 1590 APOD 2003 April 7 http://antwrp.gsfc.nasa.gov/apod/ap030407.html

Open cluster: M11 APOD 2003 January 22 http://antwrp.gsfc.nasa.gov/apod/ap030122.html

Planetary nebula: The Helix Nebula NGC 7293 APOD 2003 May 10 http://antwrp.gsfc.nasa.gov/apod/ap030510.html

Cat's Eye Nebula: APOD 2002 March 24 http://antwrp.gsfc.nasa.gov/apod/ap020324.html

Artist's interpretation of the white dwarf star H1504+65: from "Naked White Dwarf Shows its Dead Stellar Engine" http://www.space.com/scienceastronomy/mystery_monday_040705.html

Stars in Scorpius: from APOD 2012 September 12, http://apod.nasa.gov/apod/ap120912.html

Stars and dust in the Milky Way: from Astronomy Picture of the Day, 2003 September 28, http://antwrp.gsfc.nasa.gov/apod/ap030928.html

Spiral galaxy: M51, from the Hubble Heritage Project, http://heritage.stsci.edu/2005/12a/index.html

Group of galaxies: the Hickson Compact Group HCG 87; APOD 1999 September 6 http://antwrp.gsfc.nasa.gov/apod/ap990906.html

Galaxies: Galaxies in the Hubble Deep Field South, in Fornax; APOD 2003 June 25 http://antwrp.gsfc.nasa.gov/apod/ap030625.html

Billions of clusters: Hubble Ultra Deep Field, from Hubblesite http://hubblesite.org/newscenter/archive/releases/2004/07/

Sheets and bubbles of galaxies: from the Millemiun Simulation http://www.mpa-garching.mpg.de/galform/millennium/

Nine planets: Solar system montage, PIA01341 from the NASA Planetary Photojournal http://photojournal.jpl.nasa.gov/catalog/PIA01341

Planets and sun to scale: from IAU Draft Definition of "planet" http://www.iau.org/public_press/news/detail/iau0601/

Scale model of the Solar System: from "The Thousand-Yard Model" by Guy Ottewell http://www.noao.edu/education/peppercorn/pcmain.html

Updike: quoted in Hirshfeld, "Parallax: The Race to Measure the Cosmos", p. 68

Quasar image: from "High Redshift Quasars in the ING Wide Field Survey", 
http://www.ing.iac.es/PR/newsletter/news4/science1.html

Distances of stars in the Southern Cross: from "Distances" by Peter Caldwell, http://users.netconnect.com.au/~astronet/dist.html. Used with permission

Electromagnetic spectrum: from "Imagine the Universe" http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

Wavelength: from "Cool Cosmos", http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/emspec.html

Wavelength as a function of temperature: from Astro11230 by Richard McCray http://cosmos.colorado.edu/cw2/courses/astr1120/text/chapter1/lesson1.html

Atmospheric transmission from Wikipedia: Space observatory http://en.wikipedia.org/wiki/Space_observatory. Current astronomical missions: from http://nssdc.gsfc.nasa.gov/astro/astrolist.gif. NASA Great Observatories: from Chandra http://chandra.harvard.edu/resources/illustrations/elec_mag_spec.html

Wavelength scales: from "Cool Cosmos" page, http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/emspec.html

Multi-wavelength images of Cen A: from "Cool Cosmos" 
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_museum/cenA.html

Prism: from "How Stuff Works" http://science.howstuffworks.com/question41.htm

Blackbody spectrum: from "Explorations" by Thomas T. Arny, Fig. 3.6 http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0306.html

Fraunhofer lines: from "Quantum Physics" by S. Raychaudruri, http://home.iitk.ac.in/~sreerup/BSO203.html

Types of spectra: from "Explorations" by Thomas T. Arny, Fig. 3.15 http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0315.html

Doppler shifts: from "Explorations" by Thomas T. Arny, Fig. 3.18 http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0318.html

Red and blue shifts: from "Above the Skies: An exploration into our universe through animated spectroscopy", http://hea-www.harvard.edu/~efortin/thesis/html/index.shtml

Hanny’s Voorwerp: from APOD 2008 June 25 http://apod.nasa.gov/apod/ap080625.html and 2011 February 10 
http://apod.nasa.gov/apod/ap110210.html. Explanation sketches from HubbleSite release STScI-2011-01 
http://hubblesite.org/newscenter/archive/releases/2011/01/