"C" wire in wall! Where does it go in air handler/furnace?
If there's a C terminal on your board (can't tell from picture), use that.
If you have an air conditioner, it will have two wires that connect back to the thermostat (usually via the furnace): so long as one is going to R, the other will be C and you can splice into the same spot.
Lastly, you can splice into a wire from the transformer. You'll need a multi-meter to figure out which one it is (measure 24V between the C side of the transformer and the R terminal). 


It does look like common is grounded to the chassis, so you can also use that to identify the side of the transformer -- find the side that has a wire connected to the chassis.


It'll be one of the bottom two terminals:




Test

You should use the multi-meter to test -- no matter which method you use -- before you actually splice into anything. 

With the furnace powered on but not actually running (fan off, heat off), you should measure 24V AC between the R and the place you're hooking into.