I created a Github organization account. Anyone making useful contributions can be added over time to this organization. For pioneers, this may be relevant for the (possible) overlapping stuff.
OpenSettlers works by defining abstract and concrete objects, mapped 1:1 with the game. As such, UI objects use icons/images with the same name. For example, there's a Resource interface with Sheep, Wheat, Timber, Ore, Clay, Gold et cetera concrete objects. As such, they all have a file with a .png extension Wheat.png, Timber.png et cetera.
To truly benefit from each other working towards a common goal (creating a playable implementation), I believe the best thing to do is to split the project into smaller parts which can easily be re-used. For graphics, this means there will be three main areas: 2DRaster (png images), 2DVector (svg images) and 3D (models and textures).
I do realize this 'forces' naming of ingame elements to the names used in OpenSettlersGraphics repo. I consider this a good thing, as it kinda standardizes a naming scheme. See the initial commit as a first proposal for names, I'm very open for sensible changes.