PediaIcons.txt is a different issue than the Civilopedia. Yes, the names are confusing, I apologize, I did not choose them.
The main thin you will need to watch for in issues related to PediaIcons.txt is to make sure you understand what version of the software you are playing. The PediaIcons.txt file is different for Civ3v1.29 and for the PTW versions.
Using an incremented PediaIcons.txt file will not cause your game to crash of the you can say that everything in the original or base PediaIcons.txt file is included as a subset of the new file. In other words, if you only add things then no crashes occur because these added things just become unused references.
Unfortunately the way this got implemented to PTW it is 10x more complicated than it should be and there are examples in the game where it was implemented "properly". Properly just being a different way chosen by Firaxis that does not require several external and undocumented changes.