You can do this now!
Select the cell you want to add a note to. Tap in the blue area and select Note from the menu that appears.
A text entry screen will appear. Type in your note and tap OK.
Finally, found a way to do it (while still using a draft).
On the iPhone,
click the as usual to insert the image ; however, this is the default, and the image is loaded as part of the html mail, so, ...