Thus, they can be copied to another location. On Mac OS X 10.8.2 Mountain Lion, they exist as files that are a mix of jpegs and tiffs on my Mac. If you are running Mac OS X 10.4, my memory is failing me as I don't recall if the user picture was stored in NetInfo or not.

The resolution can vary, depending on the original size of the photo and what version of the operating system you're using when it is saved. This can be read using the dscl command ( dscl. When first created an account contains a 'Picture' attribute in their user record that is a path to the image in question. Mac OS X 10.5+ stores user's account pictures within the Directory Service with the exception of an account that has not modified their user picture from when first created.