I think that its just .dds (texture), no other files are involved, except of course how it acts (spins around, flashes, etc) So I think there should be a folder for guild effects and 1 info file on how it acts, probably on the script folder.
I might look for it if I am bored ;)
EDIT:
Ok, I was bored :D
Source:
data\texture\ui\game\theme01\guilditem.dds
Pointer:
data\script\ui\com\imagetable.txt
Enjoy ^^