Step 1: Start an Instructable
To add comments you need an interface which allows you to do so. So far the only way to do so I know is within the instructable editor. So go and create an 'ible!
I prefer to have a single draft which I use for testing some html code for my 'ibles, adding notes and similar things which do not belong elsewhere and can possibly break the editor ruining your work.
Step 2: Upload Your Pictures to the 'ible You Just Created
This allows you to open them and add the comment. Since your image library is not public as far as I know (maybe staff can see it), add whatever you want there, no one will judge. Maybe.
Step 3: Click on Image to Open It and Add Notes
You don't need to add the picture to the step, you can click it right in the top bar where the uploaded ones are shown.
Pro tip: to add a note you don't need to click on the Add Note button and then drag it all the way from the corner, just click on a spot where you want the note to be. Also, if you want to maintain proportions of the note area - hold down shift while resizing it.
Step 4: Go and Add Your Pictures to Comments
You should be set now, but just in case your draft didn't save automatically - click the Save button in editor.
Head to the 'ible you wanted to comment on and select Add Images under the comment, then select Image Library in the interface which came up and select the pictures you want to add to the comment. They will have the comments in the places you put them in the draft. The comment you see in the picture was made on this 'ible in case you wonder.
You may wonder if you can remove the pictures from the draft 'ible after you use them in the comment to avoid mess. Yes you can, just don't delete it from the image library!
You can also make the comment first, then add the comment pictures from the library to your draft and add notes. Notes will appear in pictures on the comment as well when draft saves.
As a final note I'd like to say that I hope this method will eventually become obsolete and this 'ible just an artifact because someone will add the ability to zoom in the picture and add notes in it while in comment interface.
Now a little exercise to try it out and probably have some laughs - go find a picture which may be misleading without context, add a note to it and upload to comments here. I'll start in a little bit!