Open the scenario PediaIcons.txt

A little more than a third way down find the section:

#start units
Go to the end of that section (about half way down the file).

Add details of your new unit:

#ICON_PRTO_Nature
art\civilopedia\icons\units\NatureLarge.pcx
art\civilopedia\icons\units\NatureSmall.pcx

Now, move to almost the bottom of the PediaIcon page to the section:
 
####
# Unit animations

Go to the end of that section.
Add details of your new unit:
#ANIMNAME_PRTO_Nature
Nature

Save PediaIcons