To add a comment to a image, click the 'Add note' link. A yellow box should appear. You can move the box around by dragging it from the middle. You can resize the box by dragging it's edge.

Type your comment in the box and click 'ok' to save. You can edit your notes by clicking on them again. Once you exit or refresh the page your notes will be saved permanently and you won't be able to edit them.

(Image by activeside)