I think one of the biggest problems would be the inconsistency of load times.
Say, hypothetically, you have 5 players in your group. Player #1 has a Super top of the line gaming rig, and a 50 Megabit "
Ultra Lightning ++ internet plan.
His loading time is going to be on the order of seconds, so it'll flash past so fast that he might not even notice your fabulous establishing shot of the castle gates, and the archers peeking out of the murder holes etc.
Meanwhile, player #2 with his 8 year old P4 sporting 256 megs of ram and his "
basic" 1 Megabit connection, is going to have all the time in the world to study that image (If the Jpeg loads before the map finishes loading)
As much as I think it's a neat concept, I think that the loading screen is the wrong place for it.
You'd be better off creating a separate page for your establishing image, moving the players there, allowing for time to study, discuss, doodle mustaches etc, and then move them on to the next map page.