Thanks for your suggestions guys. I've watched the movie "Fellowship of the Ring" hundreds of times now, and there does not seem to be any particular scene in the movie where this pose of Bormoir comes directly from. The fact that his sword is drawn, but he is standing relaxed, suggests he's just been in battle. I was thinking of referring to other parts of the LOTR story that fall outside of the movie and allow for a bit more scope to be creative.
For example, we know from the movie that Boromir, on the orders of his father Denethor, the Steward of Gondor, rides on his own from Minas Tirth to Rivendell because Elrond has summoned Gondor to send a representative to his council, and also because Boromir has been tormented by premonitions of the ring in his own dreams.
Perhaps this scene could reflect an event, not recorded in either the book or the movie, as Boromir makes the long, lonely and dangerous ride from Gondor to Rivendell? Who'se to say he didn't stop for a bit of orc slashing on the way??