Other possibilities include a capacitor, a TVS diode for transient overvoltage protection, or a reverse biased diode for reverse polarity protection. The designator next to the silkscreen outline would give you a clue if you could see all of it. I can't see from the photo if its obscured by flux or other glop, or if its missing due to board damage. If it begins with 'R' its a resistor as you suspect, otherwise 'C' for capacitor or 'D' for diode.
What type of power jack does it use? Reverse polarity protection diodes are most common if the jack is a barrel connector due to the lack of standardisation for their polarity.