I have been playing around with Virtual Cam lately. You can install it although you don't have a webcam. It will act as a real webcam. It allow you to use pictures, videos, or your desktop as the webcam and broadcast them out. Anyway, there are few softwares that support this feature.
Just lookout them and experience it:
- http://www.freedownloadmanager.org/downloads/msn_virtual_webcam_info/index1.htm (Most of them just free trial)
- http://www.splitcamera.com/download.html (Free)