I’m not sure if this is documented already. When you create a secure note from Keychain, it will accept an image or movie just as easily as it will accept text. I was able to put an image in one note and a movie in another. You can also play the movie while it’s secured inside the note.

Create a secure note from the menu bar, then copy and paste the image into the note. That’s it, the image is saved inside a secure note. You’re done.

To put a movie inside a secure note, go to the file location on the Finder to locate the movie. Drag the movie to the body of the secure note.

[crarko adds: I’m not sure if it’s documented either. This is a fairly old submission in the queue, so things may have changed recently.]

Source: Mac OSX Hints