If it's for your own private use, you can use any image that's a power-of-two on a side.
If it's for distribution as part of an Addon, you have to make sure that its license allows redistribution.
Whether you make your own (from something you painted or photographed) or use someone else's (modified or not) depends on what you want to use it for. There are quite a few programs available which can be used to create Earth-like surface texture images, for example, some better than others. Also, the USGS provides maps of various moons and planets free.
Some planet surface texture generation programs are listed at
http://www.lepp.cornell.edu/~seb/celestia/#7.1.2
p.s. I trust you've taken a look at what's available on the Celestia MotherLode at
http://celestiamotherlode.net/