Some good sources are (these will also be listed in /usr/share/doc/xplanet/README.Debian)

If no image is specified with the -image option, xplanet looks in the current directory first, and then in the directory specified by command-line option -mapdir (if used), and finally in the directory specified at compilation time (/usr/share/xplanet/images for Debian) for the file to use as the day map. This file should be named body.extension, where body can be mercury, venus, earth, moon, mars, jupiter, io, europa, ganymede, callisto, saturn, titan, uranus, neptune, or pluto. The extension of the file by default is jpg. After installation, it is suggested that you should get planet maps from any of above locations and install it in some directory of your choice, say $HOME/xplanet and use the -mapdir option to point to the directory that contains the planet day maps.

To start xplanet in background add this to =~/.xsession=

xplanetbg -vroot &