How can I rebuild an image field with values in an Ajax callback?
You cannot alter the form in the Ajax callback. Instead, you must put a check in your form if an Ajax action is triggered and if so, set the default value.
I did this once in D7 and managed to get it to work by doing the following:

Give the Ajax button a name attribute.
Somewhere in the form, check if $form_state['input']['_triggering_element_name'] is set and when it matches the name of the Ajax button, alter the $form.

In D8, I think you can get the clicked button with $form_state-&gt;getTriggeringElement().