Why are pictures locked in shell books?

I showed Bloom to a local person a couple of days ago. We started translating a pre-school maths book called How many?, that teaches the numbers 0–6. On one page there were four pigs, and we both agreed that you couldn’t use a picture like that in a Muslim country! But when we clicked on the picture to see if we could delete it and replace it with another, we were told that the pictures were locked:


Yet the copyright page says:

Licensed under CC-BY-NC 4.0.
You may adapt and add to this work.

There is no “ND” in the licence, so why aren’t we allowed to supply a culturally-appropriate picture?

Search terms:
shell book template book

If you look at the bottom of the section with the page thumbnails, you
should see a padlock icon to unlock/lock the book.

Click the padlock icon to unlock the book for modifying.