Unearthed Outdoors has made its True Marble 250m-resolution satellite imagery available for download under a Creative Commons licence; their 15m-resolution imagery costs money. The files are available either in GeoTIFF or PNG format and range from 32-kilometre to 250-metre resolution. At higher resolutions the world must be downloaded in pieces; even then, the GeoTIFF files can exceed 500 MB. Via La Cartoteca.