It's a lythograph that the filmmakers received permission to use. The original painting is titled "Vive L'Empereur," an oil on canvas by Edouard Detaille from 1891 depicting the charge of the 4th Hussars in the Battle of Friedland (1807) when Napoleon defeated the Russian army. The original apparently is in the Art Gallery of New South Wales, Sydney.
I know about it because my sister and her husband recently purchased it at an antique shop (it's possibly signed by the original artist) and then were contacted by the production team who had researched it and apparently really wanted to use it in the film. They arranged to borrow it for their scene and then returned it to my sis. I think the story is pretty neat, and I wonder how it will be used in the film.
Maybe it's nothing more than "blink and you'll miss it" set dressing, but how cool would it be if it was prominently featured in the movie? Maybe it will be hanging on a wall in Wayne Manor during a pivotal moment. Maybe Selina Kyle (a.k.a. Catwoman, known for her catburglaring ways) pilfers it. Maybe it plays some other minor (or crucial) role in the plot. Maybe it's just in the background and no one will notice it. Maybe its scene will end up on the cutting room floor.
I'll keep my eyes open for it when The Dark Knight Rises hits movie theaters in 2012.