No that is a picture of your power supply, a transformer is just two or more coils wound on the same core. It needs a lot of other stuff to turn a transformer into a power supply.
If you have an external power supply it is most likely that it will have two outputs marked + and -. The one marked - should be connected to the ground of the Arduino.
Do not confuse (but I know you will) the ground, as in protective ground that is actually connected through your water pipe or stake in the soil, with the common ground of a circuit.
See this post:-