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Hi Paul,

For satellite images of the Earth, I highly recommend the Earth 
Observatory site and NASA's Visible Earth:

http://earthobservatory.nasa.gov/
http://visibleearth.nasa.gov/

All of these images are preprocessed and available in common image formats.

If you want to get more ambitious and start making your own images from 
satellite data, try these sites:

http://glcf.umiacs.umd.edu/data/
http://rapidfire.sci.gsfc.nasa.gov/

The Global Land Cover Site has a great set of Landsat images which go back 
to the 1970s and are available for most areas in the world.  The MODIS 
rapid response site allows you to search for current and historic MODIS 
images (from the Terra and Aqua satellite) and then order this data from 
the USGS Land Processes Distributed Active Archive Center (DAAC).  In both 
cases, you will be able to download individual image bands, which you can 
then combine to produce true or false color images using Photoshop.  
Processing the MODIS images also requires the MODIS Swath Tool, which is 
also available at the DAAC.

Although making your own images may seem intimidating, I highly recommend 
going this route.  The learning curve is a bit steep, but the rewards are 
tremendous.  You will be able to produce current and historic images of 
almost any region on Earth.  You will also have access to much higher 
resolution (200 - 300 MB).  Once you get the hang of it, the exhibit 
possibilities are endless.  One exhibit component we are currently 
building focuses on deforestation and urban growth.  We were able to get a 
series of Landsat images of the Amazon basin from the 1970s to the 2000s 
and a series of images of Las Vegas from the 1970s to the 2000s.  These 
type of images are not freely available preprocessed.

These sites and techniques were introduced to me at a data processing 
workshop hosted by the American Museum of Natural History and led by Ned 
Gardner.  They have some great remote sensing resources on their website:

http://geospatial.amnh.org/

Please feel free to contact me if you have any questions.  I also 
recommend talking to the folks at AMNH.  They are great!

Scott

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