You can power the Arduino with 12V through the Vin pin or the power jack. Bear in mind that the current that the 5V regulator can supply is limited. The more voltage that the 5V regulator must drop the less current that it can supply and stay under the practical limit of 1W dissipation. A resistor is not good for dropping supply voltage as the voltage that it drops is dependent on the current it must pass. A switching regulator to drop the 12V to 5V would be the best solution, in my opinion.