Unfortunately, there isn't an easy way to change that back.  However, in IconDeveloper go to Tools->Preferences and uncheck "Check file associations at startup."  Then open your registry and remove any IconDeveloper keys that you see under HKEY_CLASSES_ROOT->.ico->OpenWithList and .ico->OpenWithProgids.  I'm making a note to add a "Dissociate IconDeveloper" feature that will go under Tools and remove these associations so you won't have to manually edit any registry keys in the future.