You mentioned in your post,
 quote:
I could not find a way however to change the file to use a specific picture for a planet.
There is a way to do this, however it has limitations.
What you would do is open the SectType.txt file and look for entries like this...
Physical Type        := Planet
Picture Num          := 35
Description          := Moderate planet with water based oceans and carbon based life-forms.
Planet Size          := Medium
Planet Physical Type := Rock
Planet Atmosphere    := Oxygen
Then change the picture number on every entry where you have Size=Med, type=rock, atm=oxy to the picture you want for all your "earths".
Onfortunatly doing this will also cause all the med, rock, oxy planets in non-sol systems to look like that too.
BTW, the picture number in that file coresponds to the .BMP's in the \Space Empires IV\Pictures\Planets directory.
Geo