I'm not much of a mod maker, but I do know that you'll have to edit "PediaIcons" through PediaIcon.txt (I think that's what it's called). If you've added something, you'll have to add several lines. Look at the way the text is written and use that as a template. Every new tech/improvement/wonder added requires a pedia icon, but you can re-use the old ones by copying and changing the names a little.