RE: Banner of the barrows
Well of course by "barrow" they mean the excavated burial mound tombs that the GG and Wight Kings are so often procured from, so that's probably why there are many with the graveyard theme, since the typical barrow would just look like a hill.
However, one way to go would be to have a mausoleum-like door or lintel set into a hill, perhaps with some creepy trees and a big spooky moon behind them.
Although that sort of imagery is pretty vague for the onlooker, probably not very readable on the battlefield, which is the last thing you want. Obviously you want your banner to be impressive and intimidating instead of a "what's that on the banner?"
So I would go for something more specific to the Grave Guard themselves, particularly as the Banner of the Barrows is GG/BK exclusive anyway. Could simply do something along the lines of a screaming helmed skull with a latin banner beneath it, perhaps something similar stylized with elements of viking or perhaps even older celtic style art, which I would suppose would be appropriate to those earlier empire-area cultures that the Grave Guards were from. And of course, having something more about the unit than the particular effects of a magic banner serves the practical interest of being able to still use it for a unit standard without any art-based confusion.