I suspect that you are describing systems where a radio frequency transmitting device is coupled to the AC power lines - the radio frequency being modulated to carry the information. A receiver that is responsive to the RF (and not the AC, noise, etc) is connected to or placed nearby where it recieve the signal, much as a radio or TV would and demodulates it to extract the information - music, picture, data, etc.
To simply couple video information to the AC power line is possible but it would seem that the receiver would not be able to sort out the video from all the rest of the electrical energy.