How do I update the hero image on the subscription preference page?

This is very simple and built right into the theme settings.

What you want to do is edit any of your web pages in the CMS. It doesn't matter which one so long as it's one using the CLEAN theme. Then go into your Theme Settings (under the Design tab).

You'll see the section labeled System Pages. That's where you want to be.

Select the accordion that says Subscription Prefs & Password. Waaalaaa! 

Just like magic.