Thanks. I've just edited my earlier posts of Machu Picchu since
the previous versions had regions of solid black in the middle of
the photos (must have accidentally pressed the flood fill tool or
something in Photoshop...)
To create each image I shot three identical photos but at different
exposures (-2, 0, +2).
I next used Photomatix to generate a tonemapped HDR image from
the three shots. I then used Photoshop to crop and selectively adjust
the curves for different parts of the image.
I've found this site useful as a guide to using Photoshop to edit photos:
http://www.photowalkthrough.com/videotutorials.html