Tor Oera (http://www.oera.net/How2/TextureMaps.htm) has maps up to 2500x1250 for free. It also has a nice tutorial on world building.

There are color maps, specular maps (water reflects, land doesn't,) bump/displacement maps, and cloud maps.

You basically make three nested spheres. The smallest one is the planet itself. The next one out is the clouds - the cloud map is used as both a trans map and a bump map to add texture. The outer one represents the atmosphere, to give the planet that halo effect.