Adding a C wire to older 3 wire
It would appear from the pictures that the blue and orange wires (wire nutted to the red thermostat wire) should go back to the transformer for one of the two 24 volt wires. The other 24 volt wire on the transformer is the common to hook up to the thermostat. There should be 2 other wires that are line voltage coming out of the other end of the transformer. Don’t mess with those. Depending on the transformer there may be multiple taps, without seeing the transformer it would be difficult to tell which ones to use unless the transformer is marked. 

Great background on thermostat wiring is at:
How can I add a &quot;C&quot; wire to my thermostat?