Yeah, you can use the capture image button to set these manually. By default MixEmergency uses whatever icon is handed to it via Mac OS - so if you've recently opened a folder that contains one of your videos then it may already have a preview image/icon. I agree though, it would be nice if MixEmergency automatically did this for you - though I'm not certain that it is something that we would want to do while you are performing live, as it would add additional strain on your computer.