I noticed lately (not sure when it started) that if I update a movie poster, as soon as I select the image to replace the poster with and then select "save" the window that is open crashes back to the movie screen (the movie I have open) and I then have to hit the "update art" button again and then select the poster again from the poster selection screen.
In other words, it does upload the poster image, I just have to select it again after I upload it. It used to just update the poster image when I hit save. Now, it doesn't, I have to re-open the update art section again.