Sources of Map Images

The following is a list of some on-line commercial sources of image files that are suitable for use as map images. 

www.unearthedoutdoors.net 

UnearthedOutdoors resells image tiles measuring 2° on a side (49,284km² at the equator).  True Marble™ is available for purchase for latitudes between N84° and S60°. The tiles are georeferenced in WGS84 Lat/Lon projection for use in all modern GIS programs. The GeoTIFF format maintains compatibility with popular image processing programs such as Adobe Photoshop™. Mercator projection and tiled JPEG format are also available. Other formats and projections are available on request.
http://www.unearthedoutdoors.net/global_data/true_marble/purchase

WMS Global Mosaic GeoTIFF Download

This page contains links for a complete set of the raw image data contained in the WMS Global Mosaic, base resolution only. The files are grayscale geotiffs, with the proper geolocation tag information for integration in GIS. These gzip compressed geotiff files are very large, between 350 and 500 Megabytes each, and will decompress to 800MB tiffs, 28800×28800 pixels, covering 4×4 arc-degrees fo the panchromatic band and 8×8 arc-degrees for the other bands.
http://collections.sdsc.edu/dac2/telascience/telascience_data/onearth/