Note that the branding removal option only removes the visible copyright notices.
To change that image, you'll need to edit the admin template login_form. To toggle the visibility of the admin templates, you will need to enable Debug mode. Once you are done, go to the templates section of your Admin CP. You should see Admin and Email templates now visible - click Admin templates. Search for login_form.
Edit both URLs in this part of the code with your desired image:
Will this revert after a Xenforo update? Any way to keep these changes after an update since this is an update to the Master Style and not the Child Style I created?