Yup, the included planet images are seriously low res, and in many cases weak too.
For the moon, I used my Lightwave model (available from my downloads, link below), to render a higher res custom planet. Custom planet images have all sorts of uses, and it's handy to grab them from NASA. Nebulae also work well, and I had good results with a Death Star as a custom planet too!
(When loading the planet there's a button for 'custom' and you can link it to any image which will be additive to sky but go behind clouds).
Nick