By GCON, do you mean a description for a unit/building?
And by custom graphic, do you mean the Civpedia Icon?
If so, please follow this. While this is a generic "How to add a unit" guide, it includes information on how to add the civpedia icons as well.
I had the idea of trying to make a new Civilopedia category (for tactical advice) after what you said. This is what I tried.Add something like this in the pediaicons.txt file:
#ICON_GCON_Espionage
art\civilopedia\icons\<GCON IMAGE LARGE>.pcx
art\civilopedia\icons\<GCON IMAGE SMALL>.pcx