I have signed up for the Windows Azure Active Directory Premium Features preview. I found that once I had done this, the configure tab in Azure portal had additional options. I added some images and text to the various options and saved the changes. It said it had succeeded in saving these. The changes didn't show up, so I gave them overnight to ensure there wasn't a replication problem. The next day still no customization.
My expectation is that if I go to a SharePoint Online URL where this AD tenant is being used, that I would be presented with the customizations on the sign in page. (Please correct if this is wrong)
I went to myapps.microsoft.com and found that a squished version of my 60x280 title graphic was showing.
As a troubleshooting step, today I removed all the images and left only the text. Myapps back to generic, still no changes in the log in page.
-Frank Long