Unearthed Outdoors offers a free download of its True Marble global dataset. The catch is that the free download is of the 250m set, which is relatively low resolution — though for many more casual uses this is still relatively high resolution. Especially since the 15m high resolution image goes for $25,000 for the whole thing (you can buy smaller subsets of the images for less, however).
Even at only 250m, this is 32 images that take up several gigabytes (for $20, Unearthed Outdoors will send you a copy on DVD).
And the best thing is the 250m version is CC-licensed with an Attribution license. Use it however you like, just credit Unearthed Outdoors. Nice.
How should I use that picture?