You can create a URL for an image by uploading it to the CMS.
You can access the CMS from the Admin menu / Webcast listings area (bottom left corner of my screenshot, the file folder):
From the CMS, select a Library, such as "Registration Images"
Use the bottom left navigation to select an image from your computer, then upload to the system.
Once uploaded, you can click on the image in the preview, then select the "Open" button.
Selecting "Open" will open a new window showing your URL.
You can use then that URL to update the email messages, login and registration page as needed.