RaisingK, thanks for that script.
For anyone wondering how to use it, highlight the code in that box and drag it into your bookmarks (menubar, or bookmarks bar). Edit the 'z' part in it if you want it to be assigned to another key.
Then just click on the bookmark while at the post of the image you're adding notes to, then press Ctrl-Z for a new note.
Works for Firefox and probably any browser that support Javascript in their address bar.