I'm looking for the file that corresponds to my account photo (this is also my iChat icon).
If you wish to extract a 320x320 version of your account picture you can do so by running the following command: dscl . -read /Users/short_username JPEGPhoto | tail -1 | xxd -r -p > /PATH/TO/accountImage.jpg
Since a 320x320 image would be too small for many purposes, it would be preferable to look first for the actual photo in ~/Library/Images/iChat Recent Pictures/.
The following "generic" command, when copy-pasted directly into Terminal and run without modification, creates a small file named MyAccountPic.jpg on the current user's Desktop:
dscl . -read /Users/`id -un` JPEGPhoto | tail -1 | xxd -r -p > ~/Desktop/MyAccountPic.jpg