Pending responses from the experts, I would think you would want to place a light bulb as low as you can safely get it, since heat rises. If your extension cord will handle it (preferably 12ga, but I think 14ga will work if it's not too long), you can pick up a 1250/1500W heater with internal thermostat real cheap. The link is just one of many examples:

Here is a link that might be useful: Heater

katefisher-
That's not a silly question. A regular lightbulb, even a 150 watt bulb, would not likely be enough to maintain the temp above freezing, especially if the cold weather lasts more than a day. I agree with billala on getting a heater. Mine is an oil-filled, radiator style heater. It has two settings (750 W and 1500 W), and a built-in thermostat. These can typically be bought at Home Depot, Lowe's, etc., and they are not expensive (mine was $50).

If the outside temp is in the twenties, and your greenhouse is sealed well, you would be able to keep that size greenhouse at a comfortable temp above freezing (probably even on the 750 W setting). The thermostat is an important feature so you can maintain an even temperature instead of running out and checking/adjusting all the time as the outside temp changes. Also, with a heater of this size, I agree that a 12 gauge, outdoor extension cord is a good idea. Finally, take care that the plug end on the heater is elevated off the ground so that it doesn't end up sitting in water and pose a shock hazard, and that the heater is on a stable surface (like a board).

Something else you can do to retain heat in your little GH and burn less electricity, at least at night, when temps are at their lowest and lack of light is immaterial anyway:

Get another tarp- like the one you are using to cover your firewood, but large enough to completely wrap the GH and all it should take to hold it in place in such a sheltered site is a long bungee around the bottom. If you have any old blankets, or better; light fluffy comforters on-hand, lay one over the top of the GH, under the tarp. The roof is where most heat is lost.

But leave the tarp loose over the GH, securing it tightly only around the very bottom. The air trapped between the GH exterior and the tarp is the best insulator of all.

These changable days of early spring are a challenge for all of us with GH's, large and small. Warm one day and cold the next, but your little portable should be quick and easy to cover and uncover evenings and morning daily if need be, until the last frost.

Cheer up, it's almost spring.

birdwidow has a good idea...especially since conservation is always cheaper than burning more energy. The only thing I can think to add is to make sure that any addition to the structure, like a tarp, has a smooth enough surface, without wrinkles or major seams, to allow snow to slide off, since it looks like the frame might not support much of a snow loading.