There doesn't appear to be an easy way to do this in the Flowplayer api. I think the best approach would be to handle the click event on the thumbnails, change the splash image and unload the player.
Example
var api = flowplayer();
$('.thumbnails').click(function (e) {
e.preventDefault();
// Store the splash image url in a data attribute.
var splashUrl = $(this).data('splash');
//Replace the splash image.
$('#player').css('background-image', 'url(' + splashUrl + ')');
//Unload the player.
api.unload();
});
When the user clicks on the player load the new video using api.load()