How to change background image for email verification page before redirect

We’re using the classic login with a custom domain. After a user signs up, they click on the link in their email to verify their email address. This takes them to a page on our custom domain:

https://mycustomdomain.com/u/email-verification?ticket=SomeRandomString

This page is currently showing a background image that is now out of date. How do I change this image?

I’ve been through all the branding and customization settings I can find on the dashboard but can’t for the life of me find where this image is being set.

I think we might have set this when we were originally using the New universal login, but we’ve since switched back to the old one.

Any ideas?

I should also say, this is the only location where this old image appears - everything else is correctly branded with the new images.

The only solution to this I have found is to temporarily switch to New Universal Login, remove the background image there, and then switch back to Classic login.