Symptoms

When the registration wizard is running, it shows boxes with a black x, instead of the usual spinner and green ticks.
 

Diagnoses

The "Forms Image" SmartBox SmartObject had a list of users in its security settings (K2 Workspace > Management Console > SmartBox > Security > FormsImage).

This will prevent users that are not in the list from executing the smartobject methods. Likewise, the account used to run the K2 blackpearl for SharePoint registration wizard wasn't in the list.
 

Resolution

After adding the account running the registration to the security list, the images were able to load fine in the registration wizard.