New Thermostat Ecobee3 to Old Galaxy GG-75 boiler - Needs common wire
From the pic, I'm guessing the transformer is the black device under the junction box. Looks yellow and purple coming off of that, you're connecting to the yellow, so the purple would be the common. To be sure, you'd want to connect a multimeter and verify the voltages to ground. It would also be helpful to find the wiring diagram for your boiler to be sure you're interpreting these wires correctly.

Assuming purple is the common, you'd run your wire to that, preferably through that wiring block where the other thermostat wires are connected on the left. There's no issue connecting the common to multiple places.