Asset Bank supports adding custom images for the Login/Registration screens, Share by URL and Published Lightboxes backgrounds.
These are large images that are displayed behind the dialogues, each time a user starts a new session then a random background image will be selected from the set of available ones and that image will be maintained for them during their whole session (i.e. anytime they visit the login page or registration page during the session they will see the same background image - it's only if they start a new session that they might see a different background image).
The Admin > Content > Featured > Manage login/registration backgrounds » section lists all the background images currently in the system, shows a thumbnail of each and details their padding.
It offers the option of deleting, editing or previewing any of the background images. The preview link will open a new window showing the background image in place on the login page.
Adding or editing a new background image takes the user to a form that allows them to select the image to be used (it should be wider than 1600px) and specify the top and left padding to use for the login and registration dialog boxes when they are presented above the background image. Moving the login position allows you to ensure it does not cover up any specific part of your image, e.g. your logo or other important aspects.