For the icons, there is the Icons_Planets.tga file, which you'll have to edit, along with 4 .brush files, referencing the new icons.
You will definitely need to edit the strings file, otherwise your info won't show up.
I added colonizable moons to mine, the most time was spent on the icons.
The easiest way I can tell you, is to open the icons_planets file in PS, look at the top-left corner of a given icon, note it's coordinates (x,y), then find that particular icon in one of the brush files, to see how it works.
What I did, was made new icons, wrote down their coordinates, then copied an entry from the brush file (did it with all 4), then changed the coordinates to match the new icon(s).
Once you look at the icon template and the brush files for a bit, it's pretty self-explanatory.
Just don't forget to change the count number at the top of each file to correspond with the number of additions you made to the file.
For reference, in the brush files, the data per icon is [XX , YY , XX, YY], where the first XX/YY set is the location of the top left of a given icon, and the second XX/YY is the size of the icon. For instance: [456 , 128 , 50 , 50]. The top left corner is at 456px right of the top-left of the whole file and 128px down, where the icon itself is 50px X 50px.
