I am probably not explaining that well, but for an example...
The JW FLV player from longtail
has the ability to add "callback" functions for certain state changes within the flash player. So you can trigger events on track change, volume change, track scan, track buffer, play, pause, playlist change, etc.
The way to do that with the JW FLV player is something like this:
The "onReady" or "on<whatever>" items are based on a list of available triggers built into the player (you can check the documentation if you need it) and the code afterward is the script that will be executed when the player's event trigger is fired.
If the player you are using has similar functionality then it is as simple as adding a callback function in the player's initialization so that you can trigger the image effect once buffering ends and playback begins. If not, then you might consider switching to the JW FLV player.