Hi,
Your URL require a user account to login to access it.
So I'm not sure which image you're talking about so I can't say much so far as I don't know which image you're talking about exactly.
Could you provide a test user account so that I could check on the situation ?
For the emails, if you go in the menu System>Emails, and edit an email, you can see the "template" setting with an edit icon.
If you click on that icon, you can modify the template used by that email in a popup. And there, you'll see the img tags of the images used for the header, etc so you can change their path to point to other images your previously uploaded via the joomla media manager.