The Caravel Mod

[Stazi]

You can make your own folder and let the game know of it. Check the vikings.txt in the startpos folder. You'll find line like: <SetMapTexturesSubdir:: "vikings">
Put line like this in EARLY.TXT and copy your desired files from vikings folder to your own folder. I don't remember how the game choose the right pics for certain culture but you have something to experiment with.
---- EDIT ----
All you need is to create "Orthodox" folder inside the campmap\pieces\Units folder. I've just done it and copied all "ARMY_" files from the vikings folder. Works without problems. Byzantines looking like filthy vikings - I don't think so. But If you like it....

BTW I'm curios if we can set 5th culture. Originally there are 4: CHRISTIAN_CULTURE, MUSLIM_CULTURE, ORTHODOX_CULTURE, PAGAN_CULTURE but I've found something called GENERIC_CULTURE. I don't know if it's fully usable (like CUSTOM0-9 entries for units) or just a set of base parameters but it's worth to check.