The basic parameters took a little bit of time to render on my system, but not long enough where I felt the need to decrease image size or sampling rates to speed things up. To speed up the render-view results process though I created a script for each pbrt scene file which would render the image with pbrt, convert it to a tiff file, and then open that tiff file. This cut down on typing and made it a lot easier to tweak parameters and see the results.
I had to play with three parameters to get the first image to appear like the example: coneangle, color and points from. From the image, it is clear that the light is below the head model and points up towards it. The difficult part in recreating the image was getting the light intensity right as well as where it illuminates (the neck was the tricky part). Adjusting the color allowed me to make the light less intense (rather than having to move the light position), and then decreasing the cone angle let me get the neck lit correctly, as well as to keep the background dark.
The hardest part with the second one was getting familiar with pbrt's transformation mechanism. I experimented with positioning the light both using the camera coordinate system and without it. After I was familiar with how to transform the light, I simply put it on the right of the face and then adjusted it's position to try to match the sample image as best as possible. I also experimented with changing the radius to better approximate the sample image.
For the third configuration I reused my lights from the previous two assignments and then made changes to color and position parameters to try to match the sample image as best as possible. I had to move the spotlight around until I got the highlights on the left side of the face, as well as the shadow on the nose and neck looking close to the orginal.
This one was pretty straight forward after the previous three. The hardest part was just the first light, since at first I thought we should use a spot light and tried to get things to match up, but eventually realized that an area light was better. I rotated both area lights towards the object to try to get the lighting to match a little better.