DeviceRead["RaspiCam"] should return an image on the Pi, and I've checked that it does for me. If you encounter issues with that, please let us know here.
$ImagingDevices , ImageCapture , CurrentImage , etc. are not supported on the Pi. Instead, the RaspiCam is supported using the RaspiCam driver, documented here:
http://reference.wolfram.com/language/ref/device/RaspiCam.html
Hope that helps,
Lambert
