this paramter is managed by the applet (this is no CaptainCasa parameter). We have the experience that you need sometimes to refresh the Java client buffer in order to see an updated image...
CaptainCasa has an additional parameter - this is the image that appears within the screen that comes up, when the applet got started. THis screen can be customized as well, but you currently cannot pass a URL reference. Details on how to update this screen are given on request only, because this is only allowed to non-binary-license owners...