Now I understand what you mean. I tried to deactivate the flash plugin/module and I don't get any fallback image. There's also no image defined in the source code and tbh it doesn't surprise me because I never saw a image fallback for flash content. That said I could only find one thread @stackoverflow: http://stackoverflow.com/questions/5801615/fallback-image-for-flash-object which might help you. Basically you can try to set a background image for the lightbox div. You can set it with css code like:
background: url('http://www.webdesign-lartigue.ch/images/no-flash/Hybris-no-flash.jpg') no-repeat 0 0;
I'm not sure if it works though - the chances are good that it doesn't...