First you need at least two photos which belongs in some way together. This could be a detail-view of a photo or an other perspective of the same location or something like that - just play with your creativity (you are a photographer so you have it!).

Then go to the "View all sizes" - page of the image you would like to include to another via a note:
How to put a picture in a picture using notes
There you find the links which are provided. Click on the link to display them (as I said this is a club-member-only-feature).

After then you could see it:

Copy the HTML - Text out of the full include-part (the second box, not just the link to the photo-page).

Then go to the photo in which you would ike to include this pop-up-window (in single-photo-view-modus).

Then click on the right, grey navbar at "More options" and select something which sound like "Add a note".
A note appears on your photo. Insert the code you had copied before plus any more sign (maybe a whitespace, but there must be even one more sign than just the copied insert-html-code).
Then it should look quite something like the following:

After saving this note (and again, don´t forget the extra sign!) there should be your first PictureInPicture appearing....
Then add at least the tag "pip" to the fullfeatured photo (not the one which you have included allready in the note at the other) and quite after that fav this blogpost so I know it´s working for you, too. :o)
Now it´s time to join and post it to the PiP ★ Picture in Picture - group. :-)