Apple - Where does OS X store the profile photo it takes when you first create an account?

The photo can be found in the /Library/Caches folder, if you haven't cleared this folder yet. Otherwise, I recommend typing:

dscl . -read /Users/short_username JPEGPhoto | tail -1 | xxd -r -p > 
/PATH/TO/accountImage.jpg

on one line in a Terminal window, from Chealion's brilliant answer here. In the above example, a JPEG copy will be saved to /PATH/TO/accountImage.jpg.

Unless your account is a network account from a server such as OSX Server using directory services (or AD on Windows or LDAP on *nix), then it gets stored in the plist file that stores the local directory services data on your Mac.

Before Lion, these files were simply text formatted xml files stored as shortname.plist in the directory /private/var/db/dslocal/nodes/users

The image is encoded as a JPEG file under the key:

    <key>jpegphoto</key>
    <array>
            <data>
            /9j/4AAQSkZJRgABAQAAAQABAAD/4QBARXhpZgAATU0AKgAAAAgAAYdpAAQA
            AAABAAAAGgAAAAAAAqACAAQAAAABAAACAKADAAQAAAABAAACAAAAAAD/2wBD

If you selected a stock picture instead of a custom one, you may be able to locate that file with the following command:

    dscl . -read /Users/shortname Picture

The system also caches more than a handful of previous pictures so you can usually go back to a favorite picture using the system preference graphical tools. If your plist is stored in binary format, you may use a command like this to dump a xml version of the binary file to /tmp:

    plutil -convert xml1 shortname.plist -o /tmp/shortname.plist