Context of the use would help in providing a better answer. UnknownSouljer already pointed out NASA being a great source for detailed satellite images, but if you want maps, ArcGIS is a possible solution. You can get a 30-day trial and use any of their template maps.
I recently put together a big map-of-the-world puzzle, and it got me thinking that some high-quality, high-detail maps of various continents and regions of the world might make an ideal set of wallpapers; they'd be fun to look at (I love geography) and I'd be able to learn more stuff (cities, borders, regions, geographic zones, terrain features, etc).
So basically, for wallpaper. And learning.
evilsofa: yes, I have that one! It's fun, but not so much useful for learning