A small excerpt from the artist (follow that link for his full post on this and other 40K pieces he has done):
I actually had to make some fairly major changes with this painting. Originally there weren’t any big banners. I wasn’t sure if the painting would become too complicated and clustered if I added banners. The Wood Elves also weren’t originally wearing any helmets, so I had to add those. I also had to change Orion’s pose and a few other minor details.
~Have at it!