The other pictures in this series seem to suggest this is happening at night. There should be some lights on the bottom of the tanker that help the receiving aircraft line up in the correct position for the boom operator. Because the air refueling receptacle is on the top of the aircraft often above or behind the pilots' heads, there are some indicators, the Pilot Director Lights, that tell the receiving aircraft to move up/down and fore/aft as required.

This is one example I found that shows what the lights look like on a KC-135:
http://www.zpci.com/success/kc135