Hi Rob,
You can't do that, because as it is now the picture images do not have an identifier on the user side. They just upload and then you will have no identifier to tell the software which one to change.
It is planned though that they will have an ID, but it is not worked out yet how it will exactly work and when will we get to this development phase is also yet unknown.
One way could be though to use the ID in the screen editor, but that makes some things complicated. Anyways, we will get to it sometime and will see how to make it...