Hello Laura,

You should have a look at the following packages as they all deal with image
transportation:
http://www.ros.org/wiki/image_transport image_transport   (do not miss the
tutorials on writing image broadcasters).
http://www.ros.org/wiki/sensor_msgs sensor_msgs  contains the basic data
types for image.

I do not know about the S-video to firewire adapters.
To me, the simplest solution is investing in a 20 Euro usb webcam and use
either the usb_cam or gscam package.

Concerning the cmucam, a colleague of mine released a package for the 
https://github.com/rene0/ros-surveyor surveyor 's blackfin camera. Maybe you
can inspire from it in order to populate the image message.

Hope it helps

Raphael
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