Invisable flash movie

Is there any way to put a flash movie over an existing html image and make the flash movie transparent so you only see the things you drew in flash and not the flash background. eg. I have a image of a cityscape in html. I want to paste moving people (flash movie) over top of it but I only want to see the people, not the background that flash provides. My client wants a fast loading page so putting the cityscape picture (150Kb) within flash is not an option. Any ideas???