Playlist thumbnail format

Can anyone tell me what the best thumbnail format to use in video xml playlist is. I've tried png, it works with ie8 but not firefox. jpg works with firefox but not ie8. gif works with firefox and ie8 but not safari or google chrome. I am using flow-1. ..... http://www.rockhamptonpistolclub.org.au/joomla/index.php?option=com_content&view=article&id=56&Itemi... the last video is the problem one.
Or am I doing something else wrong.