How to use rendered file entity in view header as background image for view
Is there something the Background Images Formatter module can do for you?

The module allows you to set a background to any CSS selector(s) you can think of. Combine it with a selector built with NID-token (NID field in Views, excluded from display, and its token used in the selector you assign the background-image to), and it will probably save you some templating headaches.