I know it's all a question of more data and bigger servers and more money to be spend, but I think the limit of 5000 pixels is, in my opinion, a little bit too low. It was a good limit a year or two ago, but newer movie stills are quite often in a resolution above 5000 pixels (5616x3744 for example is a resolution often used in The Hobbit: An Unexpected Journey but is also used in a little bit older and less revolutionary movies as Thor). So, I would suggest:
1. Set the resolution bar at 6000 pixels or so (not too perfect, because in a year or two, this will be too low again)
2. Let the maximum resolution be history and check only the megabytes required (I think this is the best solution, because it doesn't cause more cost for the server providers, but allows to upload stills in their full resolution).
I hope this is possible 😉