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. When first created an account contains a 'Picture' attribute in their user record that is a path to the image in question. This can be read using the dscl command (dscl . -read /Users/${USER} Picture).
If a user has modified their user picture at any time the account picture (aka cropped version if appropriate), it is stored in the 'JPEGPhoto' attribute of their user record.
Original files can be found at /Library/User Pictures if you're looking for an Apple provided picture, while your personal original files should be found at ~/Library/Images/iChat Recent Pictures/. If the files are ever removed from the iChat Recent Pictures folder, it will not affect your user account's image whatsoever.
If you wish to extract a version of your account picture you can do so by running the following command
dscl . -read /Users/${USER} JPEGPhoto | tail -1 | xxd -r -p > ${HOME}/Desktop/accountImage.jpg
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.
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.
Best Answer
The photos aren't automatically transferred when you sync in iTunes - you have to use iPhoto or Image Capture to get the photos on your computer.