Got curious myself after reading your question, though i still would have prefered to not see an old thread necro'ed
that is only close to your question but not the same issue, and the modding subforum were the better place for it, but whatever.
As punishment you will have to read through how i found out how to..
..and because it's relevant for you.
So i first did my routine for such case, by searching (with aforementioned Notepad++) the localisation folder
and finding the script reference at the beginning of the line for the two examples mentioned in this thread,
but then found nothing fitting in interface nor the gfx/interface folder.
For Carolingian Hegemony it was simply "gov_carolingian_hegemony" that i had found in "localisation" and then looked for.
Didn't appear to make sense that i found nothing. Not yet knowing that the file i looked for had no name refering to government.
Then i got the idea to search the "common/government_flavor" folder for it and voila there it was including the reference for the interface folder.
government_flavor = {
name = gov_carolingian_hegemony
graphics = GFX_evt_rival_rulers
Now you can search the interface folder for "GFX_evt_rival_rulers" to find the exact graphics file name and path.
You can circumvent searching the interface folder, but it's not realy worth it. Assumptions can be misleading.
The result would have been
spriteType = {
name = "GFX_evt_rival_rulers"
texturefile = "gfx\\event_pictures\rival_rulers_cm.tga"
There you get the exact folder, file name and directory.
So in short
1) localisation folder - for the in-game text
2) common/government_flavour - for the reference as in localisation
3) interface - for the reference as in government_flavour
4) You now got the directory and can look it up manually or via graphic program.
You're welcome.