The only thing that confuses me is how can you prove that this is supposed to be a painting of Gondor soldiers or that they're at Osgiliath? The only symbol I really see is what I take to be a Gorgon face on that shield, and we can't really see anything of the setting itself. Seems like you could say this is any classic/medieval/fantasy battle you like, right? It really is a beautiful piece though, so don't think I'm having a go. It's been ages since I've read the books, so maybe I'm off.